the bug about player

Sometimes,I play the world with my friend.But He says all the things in his bag disappea.And when he joins the game,he finds that he is appear at the first spawn place of the world.In other words,It's like  that he is a new player For the world ,but on the next day,He gets his old things.During these days,I know he don't switch the account number.So I think it is a bug