My family of four plays Minecraft in my son's world. He logs into his account and loads his world, then the rest of us join under our own Xbox accounts (no guest accounts).

After the 1.16.220 update today, my son logged in with no issues, but the three of us logged in to find our experience points and inventory were completely gone - including the armor we were wearing.

We spent a couple hours trying to gain some of it back, then saved and quit. When we restarted and rejoined later, the same thing happened - my son's character is intact, but our three characters are reset back to zero.

I'll do some more experiments tomorrow to see how repeatable this is, on this world, on new worlds, and on other accounts. I've wasted enough time on this today.