Server setup:

• Moodle 3.7.1+ (Build: 20190809)
• PHP Version 7.3.6
• Apache 20120211
• Linux CentOS

Client setup:

• Client Firefox 68.0.2 (64 bits) on Windows 10

Reproduce:

• Go to a user's profile edit using an account with the appropriate rights (ex: admin):
  • https://mymoodle/user/editadvanced.php?id=xxxx&course=1
• Choose an authentication method that prevents the login field from being changed (ex: Shibboleth).
  The profile field will then be set as disabled in HTML:
  <input type="text" class="form-control " name="username" id="id_username" value="data_protection_officer_2" size="20" disabled="disabled">
• Submit the form using the "Save" button.
  An error appears mentioning that the login field is required. (see attached).

Explanation:

Disabled fields are not sended by the browser in the POST (see attached). (A solution here: https://stackoverflow.com/questions/7357256/disabled-form-inputs-do-not-appear-in-the-request)