Authenticator App Prompt

Question

Recently I had a user get prompted to buy office or start a trial. This was when they were trying to use Excel. I found that her account was not signed into Office 365 account. She didn’t not remember her password. I did a password reset. I went to try to sign her in and she then got prompted to use an Authenticator app. I exited out of the process in Excel, and then tried Word. I notice in Word it said to update her sign in info. I updated her password in Word. When asked to “stay signed in on all apps”, I clicked ok. After that when I went to open the Excel file, it opened right up and no more prompt for Authenticator app. Trying to understand if MFA is not enabled for the users, why choosing to stay signed into all apps would change this, or if it did?

Thanks all!