When a player joins, sometimes(like 10% of the time) the player can not see someone and that someone can not see them(only tested with 2 people they might be invis to everyone.) When they switch worlds it is fixed, or when you do /vanish (custom command, toggles .canSee for every player) fixes it if they both do it. So the problem is I think they join and no one can see them. Nothing in my code effects this and I know this because it doesn't happen everytime. I tried adding something that for each player if they can't see eachother, toggle the .canSee but the bug still happens. Spigot version 1.12. I think it is server side considering I gave the plugin to my friend and it hasn't happened on his server yet(it is rare so maybe thats why?) And I have a boss system where every x seconds a boss spawns, if the player is not in the same world as the boss, they can not see it.