LAN World Bug

I was playing on a LAN world recently and I found that when my friend joined the world, he lost all his progress. When I look in playerdata, I found a third players information, even though he and I have been the only ones that have ever played on that world. This also happened when  20w18a first came out. It seems that every time a new snapshot comes out, the LAN player creates a new profile, but then the second time we relaunch the game, he gets his old stats back