Slimes despawn instantly

Create normal superflat world. Creative mode, normal difficulty. Set it to day time to prevent other mobs from spawning.

See slimes spawning and despawning instantly in the distance. Very rarely, one will survive and won't despawn. But still, very rarely.

Note: I haven't tested with magma cubes, but I assume it's the same issue. On our snapshot server, I haven't seen a single one in 3 hours of gameplay.