[keycloak-user] Custom Identity Brokering and First login flow: prevent username edition?

Hi, I'm using a custom IDP and I have some trouble during the first login flow, it redirects to a "Update Account Information" page (this is fine), but you can edit the username in this. How to prevent this behaviour? As the usernames are provided by the third party, I don't want people to be able to change them. Thanks, Rémy