Animal despawn from pens

My sheeps and cows have despawned (desappeared) from my pens (7*5). I fed them of course. The sheeps' pens were double sized but devided by fence doors at the half. There are 12 pens built next to each other. They're quite close to my house. I think I've done everything well and it's a bug. The version is 1.13.1.5