We're in the middle of migrating users to 365, and easily the most common issues users have is that upon launching Outlook for the first time after migration the Microsoft Account logon box shows up, and instead of having their email address listed it has their domain\username credentials.
I didn't experience this in any of my test environments, so I'm not sure what causes it. It happens to maybe 30-40% of the users I've migrated so far, however. It doesn't seem to matter if they already have a MS365 app on their machine or not (we've had some of these users using Teams prior to migration). It isn't a huge issue to resolve (I typically just need them to click on Sign in with a another account and it'll then pull in their email address automatically, though a few users have needed to type it in).
I'm wondering what sort of situation will cause this and if there's any way I can pre-emptively mitigate these issues.