On an iPhone XR (this issue has been seen with all flavors of iOS 13 and is also happening on iOS 14): I am using Outlook to access my company email (via site: login.microsoftonline.us) which is connected via Microsoft InTune so the connection and data is encrypted on my device. When I need to log into Outlook, it brings up Microsoft Authenticator to enter my Password, where I use LastPass to fill in the information. I have to select LastPass twice before the password is populated in this app (e.g. where it says "for microsoftonline.us - LastPass <username>").
After I do this, my Outlook login information is stuck in a cache somewhere and the username and password for my Outlook account is automatically populated when I need to provide a user name/password for a page in Safari or another App regardless of the domain or company. It also pre-populates the Outlook password if I am using my phone to enter a password on Apple TV using the TV Remote app.