Passive mobs despawn

When I was collected a ton of skeleton horses within one thunderstorm and tied them opposite to a harbor, one of them despawned. This also applys when I spawned 2 iron golems and villagers that have names.

Steps to reproduce: (may not be 100% accurate)
1. Spawn an iron golem or a villager or collect a ton of skelly horses
2. Go to a place where mobs don't render
3. Relog and go back to the world

I hope you guys can fix this as possible. Thanks.

The screenshot shows a skelly horse has despawned, leaving the lead behind.