Losing all inventory on log out V1.20.60

Upon logging in to my brothers minecraft world, all his items have disappeared from his inventory, and all that he has in his inventory is a map, as if he just started the world anew. This has happened also to my cousin when he logged on to the world and the same thing happened to him, my cousin connected via multiplayer. Me and my other brother play on PC and this hasn't happened to us but there is a possibility. The world is not a realms server.