Mobs are no-clipping

Put the summary of the bug you're having here

What I expected to happen was...:
i was getting loot from the mob grinder on skyblock

What actually happened was...:
and after the mobs started to noclip out idk how that happened

Steps to Reproduce:
1. build it as low as possible
2. wait for the mobs to spawn
3. they will sometimes noclip out of the box