Looking at this topic’s OP, I see that the auth overrides username setting isn’t shown as being enabled in the screenshot. That setting needs to be enabled for DiscourseConnect to override the username. It would be great if the settings page could make it more obvious that these three settings are related to DiscourseConnect:
auth overrides emailauth overrides usernameauth overrides name
Also, make sure the users are logging out of Discourse, then logging back in via DiscourseConnect. If they don’t log out of Discourse, the change won’t take effect. You can get around that issue by making an API call to the sync_sso route on the user’s behalf: Sync DiscourseConnect user data with the sync_sso route.