https://github.com/MLG-Fortress/ExtraHardMode/blob/master/src/main/java/com/extrahardmode/module/EntityHelper.java#L310

There should be no reason for a player to be in another world, we only get a list of players that are in the world we are checking before we perform the distance check, and this is all done in the main thread - my only guess is that some plugin teleported the player in an async thread...?