My friends and I share a Minecraft world and have a number of passive mobs that we keep; the biggest example being a collection of colored sheep that we named with nametags for easy wool. However, we're running into a frequent problem in which these sheep along with our tamed horses are despawning for seemingly no reason, regardless of being named or tied to a fencepost. This has happened about 3 times already and despawn in huge groups. We often open the world to find our farm extremely empty with no trace of our animals. What's strange about this is that not all of the animals are vanishing, there are usually about 5 sheep still around but all other animals in our farm disappear.
We're not sure what's causing this. The most recent mass despawning occurred after a game crash (which its own cause is unknown as well.)