I don't know if this is a bug or an undocumented "feature," but the two G&K Follower Beliefs "Monasteries" (which allows purchase of monasteries) and "Holy Warriors" (allows faith-purchase of pre-industrial land units) seem to be incompatible.
I chose Holy Warriors as my first Follower Belief. Many turns later I enhanced my religion and Monasteries was not available. I checked the World Religions tab, where Monasteries was not shown as taken. I loaded an autosave from a few turns before, gave myself some faith with Firetuner to get a prophet one turn earlier than I had before. Even with enhancing my religion on an earlier turn, Monasteries was unavailable.
Then I loaded the turn in which I founded my religion, this time choosing Monasteries. I ended the turn, then gave myself enough faith for another prophet, ended that turn, and on the next enhanced my religion. And, yep, now Holy Warriors was not available (and not taken by any other religion).
It's clear that the issue was not these beliefs being taken by other religions, so this seems to be a bug unless they are designed to be incompatible.
I chose Holy Warriors as my first Follower Belief. Many turns later I enhanced my religion and Monasteries was not available. I checked the World Religions tab, where Monasteries was not shown as taken. I loaded an autosave from a few turns before, gave myself some faith with Firetuner to get a prophet one turn earlier than I had before. Even with enhancing my religion on an earlier turn, Monasteries was unavailable.
Then I loaded the turn in which I founded my religion, this time choosing Monasteries. I ended the turn, then gave myself enough faith for another prophet, ended that turn, and on the next enhanced my religion. And, yep, now Holy Warriors was not available (and not taken by any other religion).
It's clear that the issue was not these beliefs being taken by other religions, so this seems to be a bug unless they are designed to be incompatible.