Cows escaping from fence enclosed area

Cows escaping from fence enclosed area

What I expected to happen was...:
The cows to stay in fenced area

What actually happened was...:
Cows from the inside mysteriously appear outside the fence enclosed area

Steps to Reproduce:
1. Put Cows in enclosure
2. Reproduce a few cows
3. Leave for a while and cows will appear on the outside eventually