A similar problem exists in which the Coptic Pope always converts to Shia Islam. The head of a religion who converts to another religion should be replaced instantly.
That can be fixed by just adding a religion line to the specific titles so that it can't in theory be created or held by someone who is the wrong religion.
It's also little known that there was, in fact, a muslim pope; mostly forgotten nowadays, his special understanding of christianity inspired Michel Cervetus centuries later