LAN Server resets inventory

My friend and I like to play LAN servers, however, when he logs out and back in his inventory gets reset, along with his position and stats. After inspecting the world's playerdata folder, a new player is added with a UUID of 82714d26-1bf3-3acd-a37a-093975d2ffbc, and after deleting this file, their player data is reset to what it was originally.