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.
Environment
Since the release of 1.14 and 1.14.1 Mobs and Entities have been randomly disappearing whenever logging back into a singleplayer world or if on a server when you restart or if it crashes. There has been a bug report about it here before. https://bugs.mojang.com/browse/MC-150170
The bug is happening even when in loaded or unloaded chunks.
I hope at this point the Mojang staff are aware what's been happening since 1.14 release.
Please do not mark unreleased versions as affected.
You don't have access to them yet.