I started a new playthrough, and encountered a whole new level of bug-induced frustration. Somehow beating Katrina makes me entirely unable to gain EXP, including from the fight with her - thus effectively wrecking the entire save.
At first, I thought it may be a bizarre interaction with a level-up caused by the fight itself, but a bit of grinding beforehand to hit that level first disproved that theory. For a while I also suspected it may simply be the EXP counter itself freezing, but going through a slog of a grind made it clear that wasn't the case either.
I didn't have the problem on the exact same copy of the game with a different class (Duelist), which makes me think it may be somehow tied to either the Arcanist or the use of Eldritch Tomes (specifically the variant which gives 3k EXP), except in that case I'd have sooner expected the bug to kick in right after the use of the item rather than specifically after beating a Domina. It certainly hasn't caused any issues before that, I leveled up twice after using it.