Playing as the Doux of Athens, the Byzantine Emperor converted to the Monothelite heresy about 20 years into the game. I resisted converting for awhile but after he revoked the second holding I switched over. Fast forward 150 years and I've become Emperor and just took Jerusalem so I control all five Sees of the Pentarchy. I took Rome, Antioch, and Alexandria while still only a (powerful) Doux, and so I didn't get the event pop-up and prestige for those. I didn't even know there was an event, it's too bad it doesn't fire for a vassal who takes the holding also.
But my bigger concern is that I don't have the decision to end the Schism. Is this possibly because I'm a heretic or maybe because I took control of the Sees before becoming Emperor?
Here is the Religion page:
And here is the Religion map overlay/lack of Decision button:
I just tested it by converting back to Orthodox and the Decision does appear. I don't have enough piety to do it anyway, and I also gave away the bisophric of Antioch to the Monothelite Patriarch way back (seemed like the RP thing to do) which I'll need to get back. But it still appears that the event is only for Orthodox rulers, and not heretics. Too bad.