Seelah and Sosiel left my party after Iz when i did that, so there seems to be some sort of unfinished reactivity. They definitely shouldn't wait until after Iz to leave though. Act 5 also seems to have a lot less companion interjections than the rest of the game.I killed Galfrey right in front of Seelah and no reactivity at all.
They stayed with me when I played demon and killed both Queen and Orc Lady.