When you put sheep or other passive mobs in a fence or room, they bunch up in one or more corners of the room.

What i expected to happen: Animals would walk around like normal, munching grass and such.

What happened: Sheep congregate and glitch into one another so that you cannot shear some of them.
I can confirm this also happens with cows, pigs, llamas, and horses. Not chickens, though,
I will attach a short video, but to recreate you can just add animals to a pen or room and watch their behavior.

Also sorry for short video, recording wasnt working so well.