Animals Phasing Through Fences

Put the summary of the bug you're having here

What I expected to happen was...:
Animals stay inside of fences

What actually happened was...:
They would pile up at the corners where I had placed Logs with slabs on top rather than fences and would sometimes just avoid the fence/log collision altogether. Doesn't seem to be a mob cap issue

Steps to Reproduce:
1. Make an enclosed fence area (square/rectangular)

2. In the corners replace the fences with a block and a slab on top of that
3. Increase the number of animals (I had this problem with cows and sheep, I am unsure of other mobs) until they get stuck in the corners