After playing on multiplayer for 3 hours on my friend's world, he got tired and took a nap. 4 hours later I saw on Steam that the "Join Game" option was highlighted, so out of curiosity I clicked it. Afterwards the game booted up normally and I picked a character and began going down. Once I landed I noticed that there was no base or any source of oxygen for me, and after walking around for a few seconds I sunk through the world and my game crashed.
2 hours later, my friend goes on discord and tells me that he left it in the main menu, which means that I joined the "main menu world" that you can see behind the main menu.
That was a fun experience