Mobs temporarily glitch out of fenced areas / into blocks

this seems to happen when you have a lot of them in the same fenced of area. this applies to both fences and stone walls

To reproduce:
1. Create an enclosed area
2. Fill it with a lot of mobs, up until the point they keep pushing each other
optional. hold an item that makes mobs follow you and it will be more visible
3. mobs should float outside fence/block, then teleport back in when you get close.