I tried today to get some recordings of the bug occurring, and it wasn't occurring for me. Again, I need to clarify that this is on a fresh file since our primary MP file (that the bug was occurring on) has progressed past the Merfolk Kingdom and therefore players can no longer be forcibly surfaced due to the day ending. I don't have an earlier backup of this file, so I have no way to test this specific case on a more "realistic" file. All I know for now is that the bug does not occur on the file where I slept 8 days, then dived in MP and waited until 2300 to be kicked out, or cleared trash until exhaustion and got surfaced. Both of those worked fine for host and guest.
The only thing I can think of is that either the more "mature" file has something to do with it, or a prolonged play session leads to more instability over time. Anecdotally, during an long date-night play session my wife and I also noticed some strange behavior with Rock Domino and with the guest's ability to harvest kelp, but after quitting the game and re-loading the save file, things started behaving normally again. We were going to post a separate bug tracking thread for the kelp issue as Rock Domino has already been documented, but after we tried the age-old "turning it off and on again," the issue never resurfaced.
I was able to capture footage of diving to exhaustion on the mature file, but post-Merfolk that leads to a "faint" rather than a emergency surface. This soft-locks the guest (they can't move or interact in any way), and they have to wait out the rest of the day until 0200, though I assume this is intended behavior since working until exhaustion in single player forcibly ends the day, so the multiplayer equivalent would probably be the exhausted player "faints" until 0200 when they would pass out, while the other players can continue playing normally.