Horse Becomes Invisible

Twice in survival multiplayer my horse/donkey/mule and even animals close to it became invisible when I untied the lead from a fencepost. The first time my donkey disappeared for good when I relogged, the second time my horse came back.

In the picture you can see the lead on the fencepost but no horse. My friend on the picture could put a lead on the horse, but not ride it or open up the inventory.