Fences appear to be 'leaking' animals. Chickens, sheep, cows, etc are found outside of their fences seemingly at random. Have not witnessed such a leak, but leave the chunk regularly and return with a few escaped animals – but not all and not the majority. Thought it might be a reproductive crowding issue, but have now witnessed it in animals I haven't been breeding, either. (Have not seen this issue with any prior versions of Minecraft.)