The bug causes the end dimension to not save. which resets it to how it was the last time nobody was in it. Player data isn't affected, only the level data of the end dimension. I found it to occur:
-When the hosting player is in the overworld and a player connected to LAN in the end dies by falling in the void
-When the hosting player is in the overworld and a player connected to LAN in the end leaves the game (his playerdata isn't affected, allowing item duplication)
-When the only player in the end dies after all LAN players have left (at X=260 with chunks at the main island unloaded)
In each case the chunks in the end get unloaded but don't get saved, possibly because the open LAN game causes the game to thunk that other players are loading the chunks.
This bug is extremely serious as it enables item duping (Hosting player in the overworld, LAN player in the end takes items from a chest and relogs, duping the items) and it causes players to loose hours of gameplay progress.