Oh, just realized that this may be a bit more complicated than described above: these errors didn't use to appear. They only started when I went through and started addressing the JPA spec validation "errors" I had in Eclipse. Those included the fact that I had @Entity classes and fields marked as final, and that the GameRound.GameRoundPk @IdClass on this class needed to be tweaked: one of its fields was incorrectly an int, rather than a GameSession.GameSessionPk reference. As part of fixing that, I had to create the GameSession.GameSessionPk class.

Unfortunately, I have no clue which of those fixes was the trigger for this issue.