Under Account Settings > "Display Name" the field to enter a new display name is recognized as login-field to enter your credentials and thus in some browsers (e.g. Firefox) populated with saved credentials (usually email addresses) which confuses users.

See screenshot. The yellow highlight is made by firefox to indicate that is has auto filled in your credentials.

Chrome also suggests filling in credentials when focusing the field.

Thank you for bringing this issue to our attention! I can confirm this should be further reviewed and I have logged an internal bug report for our development team to investigate and address as necessary, in a future maintenance release.