When creating a new world and playing multiplayer, not me but the other player has this problem, where after playing the first time you "save and leave" as second player or third or fourth, and the main account creating the world, they "save and quit". Then getting back on the world, the other player signs in and no longer has their levels or any items in the hotbar or items in the inventory on armor on their person. The game should save everything, but instead thinks the other player is a new player and starts them over and puts them at the world spawn instead of where they saved. I have also noticed that it is usually just on his account and every other account is fine, but sometimes it happens to other accounts as well.