I have a quick because I don't enaough time to thing it through :
$mail = \Drupal::service('plugin.manager.mail')->mail('user_registrationpassword', $op, $account->getEmail(), $langcode, $params, $site_mail);
//$output = $mail['result'] ?? [];
$output = $mail['result'] ? $mail : [];
Sorry you're right! I applied a patch from this issue https://www.drupal.org/project/drupal/issues/3138528 →