Ever since the update on August 20, 2024, the Single Player mode on Nintendo Switch is completely broken. If you create a new character, you will lose all of your progress as the character will not save locally. Once you close the game and reopen it, the character will no longer exist.
Also, if you are connected to internet while playing single player, the game will crash 100% of the time when you select “Save and Quit”.
The game seems to be running on a Test Version in offline mode. The Version info when offline says 1.6.81942 - TEST. This may be the issue.