Mobs glitch out of enclosed areas after logging out and then back in.

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• I build an xp skeleton farm where mobs fall down a one block shaft onto a slab. If there are mobs on the slab when I log out and close the application, when I log back in most of the mobs will no longer be on the slab but instead running around the room where I have to kill them all off.
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• I also built a holding cell for various mobs. I had three creepers inside of one cell. I logged out and then back in later and one of the creepers was outside of his cell running around my fortress.
• I also notice that mobs contained inside of my holding cells will occasionally de-spawn and my holding cell will be empty. This could be due to them glitching out of the holding cell on login.
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• After logging out and back in this also happens with animals caged inside of my fenced off animal pens.