I am using the 1.12.0. The issue still persists. Sometimes it works, sometimes it doesnt.
I've gotten around my issue by having the Switch account log off from its MS account and relogging back in. It seems to work most of the time as long someone joins quickly. Once that's done, the game works normally as intended and anyone could join the host (Switch).
I'm trying to connect to the Switch using my PC. Whenever I start the world on the Switch, any attempt to connect to the Switch will fail. I've hard reset the Switch with the wifi and cleared the accounts under the settings tab and it has been finicky. Very rarely will it allow me to join.
I've also tried connecting the Switch to my PC and I'm having the same issue. On the PC it'll say my Microsoft account got disconnected with little warning, and the Switch user cannot connect. And sometimes very rarely it will work.
Oddly enough, when my cousin hosts a game on his PC, we were both able to connect to his game, but he's unable to join to the Switch.
Update: After trying to start a game on the Switch as the host, it will briefly appear and then disappear on the PC. After about 10-15 minutes, the Switch will lose connection from its Microsoft account. I've checked the NAT settings under the Switch and it's listed at B, so it's definitely not that.
Before the 1.12 update, the games were working without any problems, but stopped working as intended after the 1.12 patch.
This is also present in 1.12.0. Using PC and after a few minutes, I get the message on the menu. Can still access marketplace.