Lan Server deletes player data when player logs out

When I logged out of my friend's lan server, I had a full inventory and was inside of a player-created building. When I logged back in, I was at the original spawn point, my inventory was empty, and all of my achievements had been reset.

What I expected to happen was...:
I expected that when I logged in to my friend's lan server, my inventory, achievements, and location within the world would be the same as when I last logged out.

What actually happened was...:
When I logged back in, all progress that I had made was gone. All achievements reset, all items gone.

Steps to Reproduce:
1. Log into lan server. Gather items and achievements.
2. Log out.
3. Log back in and check inventory.