Upon opening the window from the Minecraft Launcher, I clicked the grey "Sign-In" button. After a few moments, it signed me in with the Username "Andrei" while my Gamertag information would not load for multiple minutes upon investigation.

This visual issue was confirmed by a friend while playing Multiplayer in a world I had generated, as it showed that any text I had sent in the message box came from "Andrei."

I have no other accounts to my knowledge, and my active account is not named Andrei.