Mobs glitching through blocks

What I expected to happen was that a large amount of chicken would stay in a small shut off area.

What actually happened was the chicken started glitching trough the walls leaving only about 10 entities in the cage.

Steps to Reproduce:
1. Spawn a large amount of mobs (at least chicken) in a 1 block space
2. Make sure there's air around the cage.
3. Load the world in one of the newer snapshots and you'll find chicken spead around the cage.

Sorry for my limited English.
I hope my message is clear anyway.