Sonarr

Third edit - Microsoft accounts did solve it, friend just hadn't shared properly. I think it would be worth putting a notice in the game that shared worlds are dependant on microsoft accounts.

For what it's worth - this remains true after enabling crossplay and logging in with microsoft accounts

As title says. We're both on steam, crossplay is not enabled, my name shows up in the 'shared with' menu on his game, yet when I try to load a multiplayer game I have no saves available.