A Microsoft file hosting and synchronization service.
I don't really have a resolution...on one of my PCs that had changed to PIN, I restored my entire C: drive back to mid-January and this got me back to using the Microsoft Authenticator code. This is a pretty drastic solution and I'm concerned additional updates may break the multi-factor authentication.
I spent about 90 minutes on the phone with Windows support and they couldn't explain why it happened or how to fix it. They referred me to the OneDrive support team - I've gotten one suggestion to unlink and relink the PC which I have done - this didn't fix the multi-factor authentication problem. I did reply this morning to OneDrive support that their suggestion didn't work.
I think you can open a case directly with OneDrive support - they only work via email - and I would suggest that. Microsoft doesn't seem to really be aware that some number of users are experiencing a broken multi-factor authentication with One Drive Personal Vault. I've seen it mentioned on Reddit too.