Historically, the Roman Catholic Church has disqualified non-Catholics from the status of "martyr":
Pope Pelagius II - Dilectionis Vestrae (585 AD)
"The beginning starts from unity, and the primacy is given to Peter, so that the Church and the chair of Christ may be shown (to be) one: and they are all shepherds, but the flock, which is fed by the Apostles in unanimous agreement, is shown to be one." And after a few remarks he adds: "Does he who does not hold this unity of the Church believe that he has the faith? Does he who deserts and resists the chair of Peter, on which the Church was founded, have confidence that he is in the Church?"
Likewise after other remarks he asserts: "They cannot arrive at the reward of peace, because they disrupt the peace of the Lord by the fury of discord... Those who were not willing to be at agreement in the Church of God, cannot remain with God; although given over to flames and fires, they burn, or thrown to wild beasts, they lay down their lives, there will not be [for them] that crown of faith, but the punishment of faithlessness, not a glorious result (of religious virtue), but the ruin of despair. Such a one can be slain, he cannot be crowned"
Notice above, that after defining all those who "resist the chair of Peter" as outside the Church, he says that even if such are killed by others for their faith, they will still endure "the punishment of faithlessness", and "cannot be crowned".
Similarly, Pope Eugene IV (1383-1447 AD) writes in the Ex Cathedra Papal Bull Cantate Domino, which was incorporated into the ecumenical Council of Florence (1445 AD):
Pope Eugene IV - Cantate Domino (1442 AD)
It firmly believes, professes, and proclaims that those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart "into everlasting fire which was prepared for the devil and his angels" (Matt. 25:41), unless before the end of life the same have been added to the flock; and that the unity of the ecclesiastical body is so strong that only to those remaining in it are the sacraments of the Church of benefit for salvation, and do fastings, almsgiving, and other functions of piety and exercises of Christian service produce eternal reward, and that no one, whatever almsgiving he has practiced, even if he has shed blood for the name of Christ, can be saved, unless he has remained in the bosom and unity of the Catholic Church.
The above reiterates the teaching of Pope Pelagius II - that even if a "heretic" or "schismatic" "has shed blood for the name of Christ", they will nevertheless be damned for not having remained in unity with the Roman Church. Therefore, non-Catholics cannot be martyrs.
And, lest there be any doubt as to whether "heretics" and "schismatics" includes all non-Catholics, note that the Church has solemnly declared many times that all Protestants are anathematized heretics, as every Protestant group rejects the authority of the Papacy, which according to Vatican 1 (1870 AD), means they are "anathema" (Session 4, Chapter 2, Canon 5, etc.). Also, every Protestant group without exception holds to multiple beliefs which incur an anathema according to the ecumenical Council of Trent (1545-1563 AD).
Orthodox Christians, also, were all explicitly and solemnly anathematized in the Council of Florence, Session 11, which states that the Church "condemns, reproves, anathematizes and declares to be outside the body of Christ, which is the church, whoever holds opposing or contrary views", after defining the Filioque, which all Eastern Orthodox Christians explicitly reject. Pope Boniface VIII (1230-1303 AD) also explicitly mentioned the Eastern Orthodox in Unam Sanctam (1302 AD), stating, "Therefore, if the Greeks or others should say that they are not confided to Peter and to his successors, they must confess not being the sheep of Christ, since Our Lord says in John, there is one sheepfold and one shepherd".
Therefore, being a Protestant or Eastern Orthodox Christian disqualifies a person from martyrdom, as they are "heretics and schismatics", which according to Pope Eugene IV and the Council of Florence, cannot be martyrs.
In spite of the above, the Catholic Church teaches clearly, in one of the decrees of the Second Vatican Council (1962-65 AD), the exact opposite - that non-Catholics are rightfully recognized as martyrs:
Vatican 2 (1965 AD) - Unitatis Redintegratio
4 On the other hand, Catholics must gladly acknowledge and esteem the truly Christian endowments from our common heritage which are to be found among our separated brethren. It is right and salutary to recognize the riches of Christ and virtuous works in the lives of others who are bearing witness to Christ, sometimes even to the shedding of their blood. For God is always wonderful in His works and worthy of all praise.
The above says that "separated brethren" "bear witness to Christ" with the "shedding of their blood". This is an explicit recognition of the validity of their martyrdom.
To establish that this is truly a Catholic recognition of non-Catholic martyrs, one can turn to many statements of post-Vatican 2 Popes, namely Pope John Paul II (1920-2005 AD), Pope Benedict XVI (1927-2022 AD), and Pope Francis, which confirm exactly that:
Pope John Paul II - Tertio Millennio Adveniente (November 10, 1994 AD)
37 At the end of the second millennium, the Church has once again become a Church of martyrs. The persecutions of believers - priests, Religious and laity - has caused a great sowing of martyrdom in different parts of the world. The witness to Christ borne even to the shedding of blood has become a common inheritance of Catholics, Orthodox, Anglicans and Protestants, as Pope Paul VI pointed out in his Homily for the Canonization of the Ugandan Martyrs.
Pope John Paul II - Ut Unum Sint (1995 AD)
83 I have mentioned the will of the Father and the spiritual space in which each community hears the call to overcome the obstacles to unity. All Christian Communities know that, thanks to the power given by the Spirit, obeying that will and overcoming those obstacles are not beyond their reach. All of them in fact have martyrs for the Christian faith. Despite the tragedy of our divisions, these brothers and sisters have preserved an attachment to Christ and to the Father so radical and absolute as to lead even to the shedding of blood.
84 While for all Christian communities the martyrs are the proof of the power of grace, they are not the only ones to bear witness to that power.
Pope John Paul II - Address to Romanian Orthodox Patriarch (May 8, 1999 AD)
7 We will cross this threshold with our martyrs, with all who have given their lives for the faith: Orthodox, Catholics, Anglicans, Protestants. The blood of martyrs has always been a seed which gives birth to new Christian faithful.
Pope John Paul II - General Audience (May 12, 1999 AD)
2 The experience of martyrdom joined Christians of different denominations in Romania. The Orthodox, Catholic and Protestants gave a united witness to Christ by the sacrifice of their lives. From the heroism of these martyrs springs an encouragement to harmony and reconciliation in order to overcome the divisions which still exist.
Pope Benedict XVI - Address to Oriental Orthodox Churches (January 28, 2011 AD)
All Christians need to work together in mutual acceptance and trust in order to serve the cause of peace and justice. May the intercession and example of the many martyrs and saints, who have given courageous witness to Christ in all our Churches, sustain and strengthen you and your Christian communities.
Pope Francis - Address to the Church of Scotland (February 16, 2015 AD)
I would now like to turn to my native tongue to express feelings of profound sorrow. Today I read about the execution of those twenty-one or twenty-two Coptic Christians. Their only words were: "Jesus, help me!". They were killed simply for the fact that they were Christians. You, my brother, in your words referred to what is happening in the land of Jesus. The blood of our Christian brothers and sisters is a testimony which cries out to be heard. It makes no difference whether they be Catholics, Orthodox, Copts or Protestants. They are Christians! Their blood is one and the same. Their blood confesses Christ. As we recall these brothers who died only because they confessed Christ, I ask that we encourage each another to go forward with this ecumenism which is giving us strength, the ecumenism of blood. The martyrs belong to all Christians.
Pope Francis - Address to the Church of Scotland (February 17, 2015 AD)
"We offer this Mass for our 21 Coptic brothers, slaughtered for the sole reason that they were Christians". These were Pope Francis' words during the Mass celebrated at Santa Marta on Tuesday, 17 February. "Let us pray for them, that the Lord welcome them as martyrs, for their families, for my brother Tawadros, who is suffering greatly", he added. In a telephone conversation on Monday afternoon, Pope Francis spoke personally with Tawadrosii, Patriarch of the Coptic Orthodox Church. He expressed his profound sadness for the barbaric assassination carried out by Islamic fundamentalists, and assured him of his prayers for the funerals.
Pope Francis - Homily (November 28, 2015 AD)
From the age of the Apostles to our own day, a great cloud of witnesses has been raised up to proclaim Jesus and show forth the power of the Holy Spirit. Today, we recall with gratitude the sacrifice of the Uganda martyrs, whose witness of love for Christ and his Church has truly gone "to the end of the earth". We remember also the Anglican martyrs whose deaths for Christ testify to the ecumenism of blood. All these witnesses nurtured the gift of the Holy Spirit in their lives and freely gave testimony of their faith in Jesus Christ, even at the cost of their lives, many at such a young age.
Pope Francis - Homily (January 25, 2018 AD)
Along the way there are also serious dangers, which endanger life: how many brothers today suffer persecution for the name of Jesus! When their blood is shed, even if they belong to different Confessions, they become together witnesses of faith, martyrs, united in the bond of baptismal grace.
Pope Francis - Address to Oriental Orthodox Churches (February 1, 2019 AD)
The seed of this communion, thanks also to your important work, has blossomed and continues to be watered by the blood of the witnesses of unity, by so much blood shed by the martyrs of our time: members of different Churches who, united by the same suffering for the name of Jesus, now share the same glory.
Pope Francis - Audience (November 16, 2023 AD)
And let us not forget our time, which has had many martyrs! Often, they are "entire communities that lived the Gospel heroically or offered to God the lives of all their members". And the matter broadens further if we consider the ecumenical dimension of their martyrdom, recalling those who belong to all Christian denominations. Let us think, for example, of the group of twenty-one Coptic martyrs recently introduced into the Roman Martyrology. They died saying "Jesus", "Jesus", "Jesus", on the beach...
Many more examples could be given of the above. And, because this has been repeated so often, and so clearly, it constitutes authentic teaching, which is binding on all of the faithful, as Vatican 2 (1965 AD) teaches:
Vatican 2 (1965 AD) - Lumen Gentium
25 This religious submission of mind and will must be shown in a special way to the authentic magisterium of the Roman Pontiff, even when he is not speaking ex cathedra; that is, it must be shown in such a way that his supreme magisterium is acknowledged with reverence, the judgments made by him are sincerely adhered to, according to his manifest mind and will. His mind and will in the matter may be known either from the character of the documents, from his frequent repetition of the same doctrine, or from his manner of speaking.
The fact that is it now dogma that non-Catholics can be martyrs is really not disputable. This despite all of them, without exception, explicitly and necessarily holding to beliefs which separate them from the Church, according to historical Roman Catholicism.
This is an indisputable contradiction in Roman Catholicism. Vatican 2 contradicts the Council of Florence. And, Pope Pelagius II and Pope Eugene IV would never have said the things that these more recent Popes have said, because they had a much more dogmatic, narrow view of the Church that excluded all schismatics and heretics, which previous councils had repeatedly defined as "all non-Catholics".