I don't understand how EntityManager#find throwing an exception "causes IronJacamar to throw" an exception.

It is IronJacamar noticing that the JTA transaction is marked for rollback only and "fail fasting" the attempt to get a DB connection (IJ has been doing that for years now). I think that EntityManager.find() used to ignore the Exception thrown by IJ and return null. We were not happy about this when we discussed this some years ago but I think entityManager.find is supposed to ignore the exception and return null.

I could be wrong but I agree that this likely changed when we merged EntityManager into Session.