Chickens get out of fenced areas

I have a squared area fenced off for my chickens and once in a while they escape through it. I've seen adult and baby chicken being outside but I do not know if the adult chicken matured outside or inside the area so i cannot say if the bug is only for baby chickens. I have not observed this issue with other farm animals, but I don't mate other animals so maybe it is a baby issue for all animals.
This together with https://mojang.atlassian.net/browse/MC-12000 is a very nasty bug. The bug just mentioned is about hit boxes of fence corners and crossings being too big so you cannot kill baby chickens (again, only obrserved with baby chickens) in fence corners.
I am aware that there are other issues like this but the ones I've seen are marked as resolved or just similar but not the same so I created a new one.

EDIT: as seen in one of the screenshots a sheep escaped my area, an adult sheep. I don't breed my sheep at the time.