I sold a bunch of stuff and made lots of money on my friends island and caught a bunch of fish but none of the bells I made or fish I caught carried over. Yet all my stuff was still sold. Is this intentional?
Someone "secretly left the island" is the only weird thing that happened. And on my island Timmy and Tommy are still separate like when someone comes over
Whenever it says someone "quietly left" it was basically a disconnect, and it will usually reset what happened on island. It can also be someone quietly left where a player uses the - button to leave, and it times out with people either in menus, or someone else trying to simultaneously leave through the airport.
The auto save can sometimes help with the "quietly left" disconnect, but I've had it where that happens, and it loses nearly an hour of progress on a friends island.
Did you sell the items through the drop-box or directly to the Nooklings? If you sold via drop-box the money should go straight to your bank account the next day. That could explain why the items weren’t returned to you, but you haven’t seen the bells.
Somebody quietly leaving could be the "-" button which has been shown to caus glitching and corruption on save files. It sounds like that might still be a bug that hasn't been fixed yet.