A Microsoft file hosting and synchronization service.
Shortly after my March 20 post above, my remaining machine (of 4 devices I use) began requiring the Authenticator app code.
From March 21 to this morning, all 4 of my devices utilized the obtained from the Authenticator app. All was wonderful!
Sadly, this afternoon, 2 of my 4 devices reverted back to only needing the Hello PIN without requiring the two factor authentication.
For anyone seriously concerned about optimal security, this is an enormous problem...and Microsoft just doesn't have a clue how this is happening or how to fix this....