I am on a server that my husband recently joined. If I play by myself on the server, it runs as normal. If I join as player 2, I lose everything on hand and in my ender chest, and then it plays as if I'm playing a totally different account. So if I log in again by myself, I have everything again. But if I log in as player 2, I'm back to what I had as player 2 instead of as myself. Every time, I am using my own live account. 

This time when I logged in as player 2, I didn't have anything that account was holding. So I lost my mending enchanted Elytra as well as a few less important items.