Mobs can glitch through fences which may be a glitch, due to the fact being the expected result should be the mob immediately being pushed away by the fence rather than staying inside the fence.

How to reproduce

Go into creative
Search for any small mobs to spawn
Then place a fence and spawn the mob inside the fence, it will glitch inside the fence.

Observations: Any small mob can glitch through fences.
Expectations: This isn't supposed to happen even if the mob is considered an entity, it should be pushed away from the fence.