The game says that the liege can determine the state religion of his vassal, however, that's not completly correct. If I vassalize an opponent, he will adapt his state religion to the one I have at the time of the vassalization. If I, the liege, decide to change my state religion to a different one at some point of the game, the vassal's state religion will remain to be your old religion for the rest of the game. Even if all of the vassal's people are worshipping your new religion, the vassal's state religion will never change. Which can cause some really annoying situations. For example, in my current playthrough, the entire world worships the same religion, including me, but my vassal won't change his state religion. Now I have to deal with tons of demands from other empires who try to claim territories of my vassal on the basis of religious oppression.

Seems like an oversight. Hopefully gets fixed soon.

Hey Yungknofu,

This seems like a bug and I would strongly recommend that you post this in our bug report section :)

Thanks a lot for the feedback !

Cheers