We are aware of an issue that appears to be resetting accounts, and the team is working to get it resolved asap.
In the meantime, as others have suggested, you can try to relink your account in the account dashboard.
When doing that, make sure you choose the account with the correct stubs and level!
Update 9:33 PM PT-
The accounts of those affected should now be restored. Please close the app completely and log back in. Thank you for your patience.
Update 10:58 PM PT-
Thank you for the reports, I'm in contact with the team investigating further for the accounts that are still missing progress.
The team is still working to resolve the issue preventing some users from re-linking to Switch platform.