When a player leaves and misses a horde event but then rejoins after, the horde warning becomes out of sync between each player. This causes some being able to sleep and some not (Becomes an issue for some mod that requires sleeping to regen health). It's able to resync itself once all players experience a horde event together.

That's not really a bug, but rather the intended way for hordes to work in multiplayer, they should sync with each other the next time there is a horde together.

That being said I do want to change the way multiplayer works, I just haven't come up with a clean solution yet.