This one keeps developing...
The summary as I understand now is that spawning and despawning is not working in chunks designated as Ocean monuments (after the blocks have been removed).
Currently I'm finding that mobs are spawning within sea lanterns during the day. I installed Soul sand under the lanterns which immobilises them. If I hit them with a sword, they still can't move but only then do they start burning in sunlight, still half in the sea lantern. If a bow with power is used, it moves them off the soul sand and they can walk around, burning.
I attech two new pictures of mobs who've just spawned).
So I am reporting mobs spawning randomly where they shouldn't, and also despawning as per originalreport below.
It must be the Ocean Monument!
Original report follows.
I am building a guardian farm and originally reported seeing guardians despawn. I've since seen a creeper also disappear at the same place. It seems to be that when a mob walks into a sea lantern and tries to climb it, it despawns (it could be prismarine brick, which is next to the sea lantern).
The pictures show one location where it happened with guardians swimming against the sea lantern but now I've removed the water, a creeper vanished in the same way in a different corner that is identical in components in my symmetrical structure.
I'll remove the lanterns and see if it persists, and update.