A site administrator is not longer able to change a user's authentication method.  After selecting a different authentication method and saving, the administrator is prompted that the Username is a required field, even when this field is populated, and the change in authentication method is not saved.  Issue has been experienced when 'LDAP server', 'OAuth2', and 'OpenID Connect' methods are selected.  This issue does not happen when the 'Manual accounts' or 'No login' authentication methods are selected.  

To test:

1. Login as site administrator.
2. Bring up a user profile, choose Edit profile.
3. Change user's authentication method to 'LDAP server', 'OAuth2', or 'OpenID Connect'.  Click 'Update Profile'.
4. Edit profile page should be returned with a message that the Username is required.  The change in the authentication method should not be saved.

The environment in which this was experienced is Moodle 3.7.1, Windows Server 2019, PHP 7.2