In the Temple of Eterra (just before the entrance to the Lotus Halls, during the quest “The Ruined Temple”), there was a boss. Mousing over this boss did not make its health bar (or any of the associated information, like name or debuffs) show up. Attacking the boss didn’t make its name (or the big health bar at the top of the screen) show up. I have monster health bars enabled, and that one (the one that hovers over the boss’s head) worked fine; it was just the UI element that shows up on mouse-over (with the name of the enemy, debuff information, and so on) that was missing.
Unfortunately, I don’t know what boss this was, because … its name never showed up. Since I was busy fighting the boss, I wasn’t able to get a screenshot either; the closest thing I have is this screenshot from just after I fought the boss: https://imgur.com/a/VVIb4VW It was large, dark-colored, and bipedal, but that’s about all I’ve got, sorry!