As I understand it, the First-and-Second Council of Constantinople in 861 was a local and not an ecumenical council.
I am not disagreeing with the statement that the RCC holds some heretical doctrines.
Canons 13-15 of the Council of Constantinople (861) permit withdrawing from communion with Church authorities who publicly preach a heresy already condemned by the Fathers or one of the Ecumenical Councils.
Since the Orthodox Church has
withdrawn from communion with the RCC, it is presumed her leadership did so for that reason.
Of course, under canons 13-15 the Protestant Reformation could also be justified!
(If the Protestants had only sought reunion with the Holy Orthodox Church, they would have been on the right course!)