Hi Jbekkaz,
Thank you for the update. The behavior is indeed strange. I would assume you are using Edge as your browser to login your Microsoft account.
Kindly chek if it is present in the saved user names and passwords:
In Edge
- Launch Edge
- Click the 3 dots, go to Settings
- Select Profiles
- Go to Passwords, check the username if present under Saved Passwords.
- If it is not, reset Edge's settings and relaunch the Browser.
- Log back in to account.microsoft.com
- Check if that username appears.
Let me know how it goes.