Animals kept in fences tend to spawn outside of them. Either on world start or after player goes to sleep.

Put the summary of the bug you're having here

What I expected to happen was...:
Describe what you thought should happen here
Animals would stay inside the fenced area.
What actually happened was...:
Describe what happened here
Animals spawn outside of the fenced area.
Steps to Reproduce:
1. Put animals inside a fenced area.
2. Either start up the world or sleep.
3. Go and look to see if any animals spawned out of the area.