In previous versions of Drupal you would see values from settings.php on the coniguration pages. For instance $conf['mymodule_setting_value'] = 'My Value'; would display "My Value" in the text field and whenever you would try make a change to the field and try hit save button the value of the field would reset to the value specified in settings.php . However in Drupal 8 overridinen configuration form values in settings won't reflect in the form.

There is an issue on Drupal.org for that and developers already found a solution and it should be fixed in Drupal core soon.