Mobs glitching through fences and other solid blocks

Sometimes with villagers, zombies, or other animals, they are capable of glitching through things like fences and doors. It was very frustrating since I was trying to make a villager city.

I expected the villagers and animals to just bump into them like solid blocks.

Instead, they sometimes "glided" through the blocks. So I had a bunch of pigs and zombies running around my city causing havoc.

Steps to Reproduce:
1. Spawn lots of pigs or other kinds of animals
2. Keep them in a fenced enclosure
3. Occasionally you might see them just float through the fence as if it were air.