We have some issues with hostile mobs stop spawning on our server.
Not like bad spawn rates because of unlit caves and such, but hostile mobs completely stop spawning, including mob spawners, and the only way to get them back is by restarting the server. (It's not enough that everyone logs out so all chunks get unloaded.)
Named mobs, item holding mobs and world generated mobs like elder guardians and shulkers remain.
Before the latest hotfix this happened once every few days or so, but since 1.18.12 it happens several times a day.