A Microsoft file hosting and synchronization service.
I have the same issue. Have used MS 365/OneDrive (Family version) for over a year. Access from 4 different personal PCs.
Personal Vault always required MS Authenticator code to open. In last week or so, two of my PCs began allowing the Personal Vault to be opened with only a PIN. I did not make this change and really need my Personal Vault to require two-factor authentication.
It must be something specific to the individual PCs as I can sit side-by-side with one requiring Authenticator and the one allowing PIN access.
This is really irritating as I specifically want the additional security...