I think the issue has cleared ... of not-always-getting-prompted-for-MFA. Thanks for following up. And I have not turned on the Enable-Remember-multi-factor-authentication option (see your link in the prior note)
As of about a week (??) ago, I started getting MFA prompts at more logins. (Couldn't say if it was all logins because I was not focused on this issue).
Some factors in the mix ... This issue occurs on my Surface Pro desktop. This started occurring more when I switched from a Windows Phone to an iPhone. It is my understanding there is a proximity setting that worked with the MFA and Windows Phone but never confirmed it. And I always get prompted for MFA on my iPhone. I have the Authenticator app on my iPhone.
Bottom line, I wanted to understand when/how MFA works. When I roll this out to the rest of the staff in my shop, I HAVE TO understand it.