Xavier,Yes, this is true and has been true for the Church for over 2,000 years. It is a general rule that holds, like the laws of physics. But God intervenes to stand the laws of physics on their head on occasion. These are called miracles. There are also exceptions to general spiritual rules or laws, unique situations that present exceptions. These exceptions, forecast by Scripture, decreed by God, are not contradictions to the general spiritual law or rule - they have the divine sanction of the King, and are part of His plan. There is no contradiction when they occur. A relevant example. Christ said about those "given" to Him by the Father in John 6:
The contrary opinion is a hypothesis only and it was the teaching of the conservative, orthodox Fathers at Vatican I who defined ex cathedra Papal Infallibility that in actual fact the Vicar of Christ can never - non-hypothetically - become actually a formal and pertinacious heretic. It was the Gallicans and other heretics who believed such had actually happened many times and the Saints absolutely fought against this and denied that it had allegedly happened to Pope Honorius and others. The Saints grant it as a possible hypothesis only but they denied that it would ever actually happen, because Jesus had promised the Faith of St. Peter would not fail.
Cardinal Billot: ""I said under the supposition of the hypothesis. But the fact that the hypothesis itself is a mere hypothesis, never reducible to an act, appears far more probable, according to Luke 22:32: But I have prayed for thee, that thy faith not fail; and thou being once converted, confirm thy brethren. For the voice of all Tradition says we must understand this verse to refer to Peter and his successors in perpetuity, and it will be professedly declared below, where the infallible teaching authority of the Roman Pontiff. But for the time being, it is assumed as absolutely certain. Now, however, even if the words of the Gospel principally regard the public person of the individual teaching ex cathedra, they must be said, as far as preservation from heresy is concerned, to extend also, by a kind of necessity, to the private person of the pontiff ... For Innocent had said earlier: “If I were not made firm in the faith, how could I strengthen others in the faith? That is what is recognized as pertaining especially to my office, as the Lord witnesses: I have prayed for thee, Peter, that thy faith not fail; and thou being once converted, confirm thy brethren. He prayed and He brought it to pass, since He was heard in all things out of reverence for Him. And therefore the faith of the apostolic see has never failed in any disturbance, but has always remained whole and unimpaired in order that the privilege of Peter should persist unshaken.” Consequently, that statement is rather in opposition to adversaries, unless they should say that by it Innocent actually means he can sometimes lack that which the Lord procured for Peter as necessary for the office to which he appointed him ... But whatever you finally think about the possibility or impossibility of the aforementioned hypothesis, at least one point must be maintained as completely unshaken and firmly placed beyond all doubt: the adherence alone of the universal Church will always be of itself an infallible sign of the legitimacy of the person of the Pontiff, and, what is more, even of the existence of all the conditions requisite for legitimacy itself. One need not fetch from afar proof of this claim. The reason is that it is taken immediately from the infallible promise of Christ and from providence." (Billot, De Ecclesia Thesis, Q. 14. T. 29, On the Legitimacy of the Roman Pontiff?
How can anyone reconcile the teaching of our Holy Father Pope St. Pius X in his Catechism with 61 year sedevacantism that claims the last 6 Popes have been heretics? It is not possible. https://www.ewtn.com/library/CATECHSM/PIUSXCAT.HTM See the Ninth Article of the Creed, the Holy Catholic Church, The Church Teaching and the Church Taught, Q.31 to Q. 72 especially. It's not possible for Christ's Vicar to ever fall into heresy, since Our Lord Jesus Christ prayed and promised that the Faith of Peter would never fail. Peter can at times be weak as St. Peter was on the night of the Lord's Passion; indeed it is necessary for Peter to be weak for this Passion of the Church to be possible, but he can never be a formal and pertinacious heretic. He may err as Pope John XXII did but in good faith, without pertinacity and therefore without losing his office. The resolution came from one of the resisting Cardinals. Perhaps it will come in a similar way today?
The Church Teaching and the Church Taught
38 Q. Is there any distinction between the members of the Church?
A. There is a very notable distinction between the members of the Church; for there are some who rule and some who obey; some who teach and some who are taught.
39 Q. What do you call that part of the Church which teaches?
A. That part of the Church which teaches is called the Teaching Church ...
The Pope and the Bishops
50 Q. Who is the Pope?
A. The Pope, who is also called the Sovereign Pontiff, or the Roman Pontiff, is the Successor of St. Peter in the See of Rome, the Vicar of Jesus Christ on earth, and the visible Head of the Church ...
62 Q. How should every Catholic act towards the Pope?
A. Every Catholic must acknowledge the Pope as Father, Pastor, and Universal Teacher, and be united with him in mind and heart.
63 Q. After the Pope, who are they who by Divine appointment are to be most venerated in the Church?
A. After the Pope, those who by Divine appointment are to be most venerated in the Church are the Bishops.
64 Q. Who are the Bishops?
A. The Bishops are the pastors of the faithful; placed by the Holy Ghost to rule the Church of God in the Sees entrusted to them, in dependence on the Roman Pontiff
65 Q. What is a Bishop in his own diocese?
A. A Bishop in his own diocese is the lawful Pastor, the Father, the Teacher, the Superior of all the faithful, ecclesiastic and lay belonging to his diocese.
66 Q. Why is the Bishop called the lawful Pastor?
A. The Bishop is called the lawful Pastor because the jurisdiction, or the power which he has to govern the faithful of his diocese, is conferred upon him according to the laws and regulations of the Church.
67 Q. To whom do the Pope and the Bishops succeed?
A. The Pope is the successor of St. Peter. the Prince of the Apostles; and the Bishops are the Successors of the Apostles, in all that regards the ordinary government of the Church.
68 Q. Must the faithful be in union with their Bishop?
A. Yes, all the faithful, ecclesiastic and lay, should be united heart and soul with their Bishop, who is in favour and communion with the Apostolic See.
69 Q. How should the faithful act towards their own Bishop?
A. Each one of the faithful, both ecclesiastic and lay, should revere, love and honour his own Bishop and render him obedience in all that regards the care of souls and the spiritual government of the diocese.
70 Q. By whom is the Bishop assisted in the care of souls?
A. The Bishop is assisted in the care of souls by priests, and especially by Parish Priests.
71 Q. Who is the Parish Priest?
A. The Parish Priest is a priest deputed to preside over and direct with due dependence on his Bishop a portion of the diocese called a parish.
72 Q. What are the duties of the faithful towards their Parish Priest?
A. The faithful should be united with their Parish Priest, listen to him with docility, and show him respect and submission in all that regards the care of the parish.
 All that the Father giveth to me shall come to me; and him that cometh to me, I will not cast out.
Omne quod dat mihi Pater, ad me veniet : et eum qui venit ad me, non ejiciam foras :
 Because I came down from heaven, not to do my own will, but the will of him that sent me.
quia descendi de caelo, non ut faciam voluntatem meam, sed voluntatem ejus qui misit me.
 Now this is the will of the Father who sent me: that of all that he hath given me, I should lose nothing; but should raise it up again in the last day.
Haec est autem voluntas ejus qui misit me, Patris : ut omne quod dedit mihi, non perdam ex eo, sed resuscitem illud in novissimo die.
 And this is the will of my Father that sent me: that every one who seeth the Son, and believeth in him, may have life everlasting, and I will raise him up in the last day.
Haec est autem voluntas Patris mei, qui misit me : ut omnis qui videt Filium et credit in eum, habeat vitam aeternam, et ego resuscitabo eum in novissimo die.
Christ said that He would lose "none" of those given to Him by the Father. The general, governing rule or law; He loses "nothing."
Now, the exception, which does not contradict the rule, the exception being an anomaly, a departure, planned and decreed by God (a suspension of the rule, a "miracle"):
John 17:12There are exceptions, foretold and revealed in Scripture, which must be fulfilled. I have no authority and do not offer my own view, but give for your prayer and reflection this: there is another "son of perdition" forecast in Scripture - 2 Thessalonians 2:3 - another "filius perditionis." Another "exception" to the general rule?We are in unchartered waters. If the Church had spoken and taught on the "exception," a "son of perdition" to rise within her and to rule her, it would have created chaos for centuries, and been self-destructive.
While I was with them, I kept them in thy name. Those whom thou gavest me have I kept; and none of them is lost, but the son of perdition, that the scripture may be fulfilled.
Cum essem cum eis, ego servabam eos in nomine tuo. Quos dedisti mihi, custodivi : et nemo ex eis periit, nisi filius perditionis, ut Scriptura impleatur.
As I said, pray, and seek guidance.
I think we are where the general rule, true and good, has been suspended. That does not make the rule a lie, as "miracles" do not make the laws of physics false.