Seeing this in 0.12.3 on Windows 10 Beta Edition. Similar issues in MCPE 10920 and 7961.
I've been experiencing this as recently as 0.12.3.
At first I wasn't sure whether the sheep on the outside of the fence used to be inside the fence, so I dyed them. That magenta sheep roaming the countryside is pretty clear evidence.
I approached the above fenced areas and found one sheep in with the cows and one in with the chickens. The left, darker fenced area (rabbits) was empty. There were a few chickens and cows running around outside of the fences.
I separated everything again and exited/re-entered the game 10 times. Over the course of those 10 reloads, one sheep (the blue one in with the cows) escaped its fence.
(Windows 10 Edition Beta 0.13.0)