I built a guardian xp farm that sends guardians to the nether to kill them with a potion of harming. However I noticed that they sometimes despawned and couldn't figure out why.

I got someone to open the portals in the overworld whilst I stayed in spectator mode in the nether and watched the guardians come flooding in. However as soon as he went came through into the nether right above the guardians they all despawned. On further testing I noticed that ALL the mobs in the nether are reset when someone enters the nether. By reset I mean that all the mobs despawn and new ones respawn around the player. There were no other players in the nether during the testing.

This bug means that any farm that sends mobs to the nether will no longer work unless someone is on the other side to prevent them from despawning.

On further testing I don't think this issue affects single player worlds. Also when I went on the server and was the only person on-line the farm worked fine.