If the field is #disabled then pass #default_value in valueCallback

I have looked at how other elements in Drupal core handle this such as the Number field, and based on this, I propose changing the valueCallback to:

  public static function valueCallback(&$element, mixed $input, FormStateInterface $form_state): mixed {

    if (!empty($input['full_number'])) {
      return $input['full_number'];
    } elseif (!empty($element['#default_value'])) {
      return $element['#default_value'];
    }

    return NULL;
  }

Patch for anyone who needs this in the meantime.

My first suggestion wasn't working and was causing an error.

rather than using !empty($input['full_number']), use isset($input['full_number']), otherwise the field will fallback to the default value if the field is left empty.