Animals teleporting through walls/fences

[Fozzel]

This might be a vanilla bug, but I've never noticed it until installing Better Than Wolves (3.72). I have a cow farm which consists of several 2x2 areas surrounded by fences, as an alternative to the laggy mess approach before automation. However, I find that whenever I log out and in again, some of the cows move around. Each pen usually has 2 cows in it, but when I log in, almost all of the time, there are some pens with 3 and some with 1, etc. I did an experiment where I replaced one pen's cows with pigs, and sure enough, when I logged back in, one of the pigs had somehow got through a fence.

Again, this could be a vanilla bug, so please disregard it if that is the case.

[Whisp]

Definitely vanilla.
If the animals are standing in the fence block, when the chunk is saved, the can appear on either side of the fence on loading.

[FlowerChild]

Vanilla. User warned. Locked.