Apple - Why does Safari autofill login information with weird username?

It' an old bug. Update safari to last version. Apple has released three updated versions of its Safari Web browser, which commonly address an issue where saved passwords would not be automatically into relevant password fields as expected. In addition to fixing how passwords are managed Apple issues fix for password autofill bug in Safari | MacIssues

Have you tried searching for the String/Username "MDoEEPg..." in your Keychain and deleted it?