The bug:
Whenever a server restarts or crashes or rejoining a single player world mobs/entities can disappear randomly even when in loaded or unloaded chunks.
Reproduce on a server:
On a server if you have a bunch of tamed horses in a fenced up area somewhere in the world, if having other mobs as well that are tamed like wolves & cats, having boats & minecarts around the world, plus item frames in areas.
Stopping your server and starting it back up a few times or even once can cause all those mobs/entities to randomly disappear.
If the server were to crash the same bug can still occur.
Please do not mark unreleased versions as affected.
You don't have access to them yet.