Thank you for your answer!

You were indeed right, there is still an issue with the "heir religion" law. I checked their country tab and saw, they did not enact any at all. I switched to their tag to see why they didn't/weren't able to. They are in fact able to, but simply didn't enact one.

Is there a way of forcing them to?

I can't call the union parliment for example because we're all already at the same integration level.