As always I had like 8-9 chickens in a flat terrain surrounded by fences when suddenly I noticed how some chickens were clipping through the fence and getting out of it. That happened when there were more chickens pushing each other in a corner.

This clipping has happening to me all the older versions , but in those older versions , the chicken couldnt exit from the fence even if clipping through it , they would just teleport back behind the fence.