If I recall, Patriarch Dioscoros was never condemned for heresy at Chalcedon, just misconduct. It would make sense that a Chalcedonian patriarch in Alexandria would have little problem commemorating a man he considered to be his predecessor, who was not anathematized or declared a heretic. Of course, people also had a different view of councils at that time, so his actions might have indicated that he agreed with the Chalcedonian definition, but personally disagreed with the deposition of Dioscoros. Who knows? All speculation aside, that is a pretty interesting bit of history, that I had never seen before.
*"The most magnificent and glorious officials said: ‘Dioscorus said
that the reason for Flavian’s deposition was that he said there are two
natures, but the definition has “from two natures”.’
. Anatolius the most devout archbishop of Constantinople said: ‘It
was not because of the faith that Dioscorus was deposed. He was deposed
because he broke off communion with the lord Archbishop Leo and was
summoned a third time and did not come.’"* (The Acts of the Council of Chalcedon, Richard Price; Session V, p. 198)
Agreed on all counts, Cavaradossi. In teaching Coptic history (or at least in teaching me Coptic history), our priests are very careful to emphasize that St. Dioscoros was not deposed for holding any heretical view (including, of course, the Eutychian heresy that is often leveled at us by those who don’t know our history, but know later polemical distortions of our history), but for procedural reasons. In failing to appear before the council, it appeared as though he admitted guilt in what was charged against him. This is, Coptic history teaches us, plainly not possible, as St. Dioscoros was under house arrest at the time-- he could not have attended the council.
"Aetius archdeacon and protonotary said: ‘As you [Paschasinus]
ordered, already before the session the deacons Domninus and Cyriacus
went from here to the most devout Bishop Dioscorus as to all the other most
sacred bishops, and bade him come here in person. But he declared that,
although he wished to attend, his guards, so he claimed, would not let him.’
…]
Dioscorus bishop of Alexandria said: ‘I am under guard. Let them say if
I am allowed to come.’
Acacius bishop of Ariaratheia said: ‘We were not sent to the hallowed
magistriani but to your sacredness, to ask you to take the trouble to repair
to the holy council assembled in the martyrium of the holy martyr Euphemia.’
Dioscorus bishop of Alexandria said: ‘I am ready to appear at the holy
and ecumenical council, but I am prevented.’
Atticus bishop of Zela said: ‘A plaint against your holiness has been
presented just now to the holy and ecumenical council by the most God-beloved Bishop
Eusebius. The great and holy council has informed your
holiness through us that you should appear and make a defence against the
charges.’
Dioscorus bishop of Alexandria said: ‘I have said once already that I too
have an intention to go to the holy and ecumenical council, but as your Godbelovedness
can see, I am prevented by the hallowed magistriani and
scholarii.’" (The Acts of the Council of Chalcedon, Richard Price; Session III, p. 44-45)*
So, you are both very correct. He was deposed for not appearing, not because he was a heretic. But he couldn’t come because he was under house arrest. Soldiers prevented him on order of Emperor Marcian.