I have a server running on my laptop. It's just my friend and I on it.

He logged out, then logged back in, and he was at spawn with nothing in his inventory.

He didn't die, and I checked the logs to make sure. I looked at his playerdata file, and looked at the revision history on Dropbox.

You can see here that it was deleted, and then a new one was added. I didn't do this, so it must have been the server.

I looked at the files with an NBT editor, and before it was deleted, it was the normal, correct one, with all of the things in his inventory, and the new one that was added had none of that in it.

EDIT: This isn't a duplicate of MC-39583. That's about your list of servers disappearing. This is about playerdata in a server disappearing.