Maybe it's not the Catholics who are misunderstanding this.
But wait - you are referring to a council of the whole Church. Obviously yopu can see then that the Church continued.

You are referring to them 'using the Roman usage' due to the council.
What's the problem?
Yes, it was a council of the whole Church that Rome called for. Asia Minor and Rome disagreed on a doctrinal issue (NewMan99 believes it only a liturgical issue). The issue apparently (earliest cite I have found) arose circa the time of Pope Sixtus.
In the council, Rome cited the letters from Palestine as support for their position. Those letters from Palestine area said, we have always followed the Roman custom and we learned it from the Apostles.
The Quartodecimans said to the contrary, Christ died on the 14th; we learned it from the Apostles.
(There is still an unanswered questions: Rome says Christ rose on Sunday. Quartodecimans says Christ died on the 14th. So, what is the assumption? The 14th is what day?).
Anyway, per this:
ANF08. The Twelve Patriarchs, Excerpts and Epistles, The Clementia, Apocrypha, Decretals, Memoirs of Edessa and Syriac Documents, Remains of the First | Christian Classics Ethereal Library
"[a.d. 180.] When Eusebius says that the churches of “all Asia” concurred in the Ephesine use
concerning the Paschal, he evidently means Asia Minor, as in the Scriptures and elsewhere.
37843784 See (Polycrates) p. 773,
supra, and Eusebius,
H. E., book v. cap. xxiii., etc., pp. 222–226. Throughout “the rest of the world,” he testifies, however, that such was not the use.
The Palestinian bishops, after the Jewish downfall, seem to have been the first to comprehend the propriety of adopting the more Catholic usage; and our author presided over a council in Cæsarea, of which he was bishop, assisted by Narcissus, bishop of Jerusalem, with Cassius of Tyre and Clarus of Ptolemais, which confirmed it. It is to be noted, that Alexandria is cited by Theophilus as authority for this custom; and it is not quite correct to say that the
Western usage prevailed at Nicæa, for it was the general use, save only in Asia Minor and churches which were colonies of the same. This fact has been overlooked, and is very important, in history."
It is obvious something is not quite right, isn't it? The Palestinian Synod, truth be told, did
not always follow the Roman custom, but adopted it. They did
not learn it from the Apostles, but from Alexandria. So, as I said, Rome alone followed the Sunday custom, citing the Bishopric as authority. The rest of the Churches followed the Apostolic Tradition.
Where did Rome learn it? The issue was over the Feast day and Fast. In the OT, however, no fast is associated with Passover. But there is a day and fast to something similar in the OT; and unfortunately, it fits precisely with your RCC assertions, much to my amazement.
Councils have been held since the Apostles to discuss events of ideas.
Certainly we know that Pascha was discussed and it took time to decide what to do about celebrating the Day [actual Day] besides every Sunday - the original Sunday Our Lord rose.
What's the dilemma?
Are you actually thinking the Roman empire was the Pope?
Or are you not understanding that Rome was already the Church everyone looked to?
Aside from there being a Roman empire?
How about St Ignatius?
Have you ever read his letters?
The Church of Alexandria is also mentioned to be second to Rome... but they do not take the place of Rome.
What I dont understand is your point to this.
Alexandria was a support (Clement again, cites the Phoenix returning to Egypt). Maybe there's some connection.
Anyway, maybe you see the dilemma? Rome's custom and the Quartodeciman Tradition contradicted each other. Scripture precisely confirms the Quartodeciman Tradition.
Thank you for thinking about this.
Like I said, I know this stuff will cut close to home, but God is faithful.
Still and all, thank God that Christ has been preached for whatever reason.