I did attempt to search the bugs, but I can't get the syntax formula up, and any attempt to search for fence related bugs only produced error. My apologies if this bug is a duplicate.

Hardcore Survival mode.

I created a series of pens to separate animals for my farm. I had only gotten my base of 2 animals each in the pens, no breeding done. I left to get materials as I am still securing the area. When I returned a Chicken had escaped, as well as 2 sheep, from their pens. I have noticed that some seem to get stuck in the fences, and this interrupts breeding. When you try to move them or get to close, it pushes them through the fence. Seems to happen in corners, although it might not be limited to corners.