When formal ecumenical dialogue between Catholics and Anglicans began in earnest in the 1960s, there was a strong ground assumption. It was this. We were glad that we had so much in common. We recognised that there were things upon which we differed. So dialogue would serve to remove the differences; meanwhile, what we had in common would stay safe. The Pope and the Archbishop of Canterbury had agreed upon a formula which made much of "the Common Ancient Traditions". Neither side would introduce new differences.
(We Anglo-papalists, of course, already agreed with the Magisterium of the Catholic Church on everything. I'm talking about the general assumption among non-papalist Anglicans and Catholics.)
Recently, there have been reports about Justin Welby. He is said to have declared that the (admirably clear) views on abortion held by a Catholic MP called Rees-Mogg are not held in the Church of England.
He could have said that these views on abortion were not universally held in the Church of England. That would have been a (depressing) statement of fact. But his actual words, reportedly, were that such views are "certainly not held within the Church of England".
That is a plain untruth. His predecessor as Archbishop of Canterbury, Rowan Williams, held very clear views on abortion. You could find them by googling 'Rowan Williams abortion'. And Williams, sadly, is still in the Church of England. Ergo ...
The Anglicans appear to have no shame about ditching those things which, a generation ago, they held in common with Catholics. And, as far as I know, Catholic spokesmen, whether in Rome or Westminster or Birmingham, never waggle a finger and say "'ere 'ere 'ere, what's going on? Are we still in dialogue or are we not?"
The ARCIC methodology would be best served if both sides were required to give, say, ten years' notice of the next load of innovatory departures from the Common Ancient Traditions which they intended to introduce.