Disclaimer: I‘m no expert, if anyone‘s a pro on this, please listen to them.
AFAIK and IIRC, heresies can appear in any religion when the fervor drops low enough, at roundabout 50%. I vaguely remember getting heresies on my „Mother Of Us All“-playthrough in Africa (African faith) and Lingua Franca playthrough (Muslim faith), but it‘s been a while.
Fervor is pretty funky in the game cause winning holy wars, for instance, LOSES you fervor for some reason, so I guess the pope likes to nuke Christianity’s fervor which leads to lotsa heresies. There‘s a mod that „fixes“ that. Unless they changed how it works, I haven‘t kept up with patches.
Hope any of this helps. Again, I‘m no expert, just wanted to give you something to go by.