8 – Theme suggestion template is not being picked up

I add template suggestion in my discount module:

function discount_theme_suggestions_field_multiple_value_form_alter(array &$suggestions, array $variables) {
    if ($variables('element')('#field_name') == 'user_discount'){
      $suggestions() = 'field_user_discount';

And I can see template suggestion on my admin form:
enter image description here

However I added template field-user-discount.html.twig (I don’t place it here as it simply the copy of classy one with change of order) in discount module templates folder and it is not being picked up, it still picks up the classy one. I cleared all the caches, both from drush and site itself. I also tried to add this template to theme templates, it is not being picked up there as well. Other templates from both theme and discount modules are being picked up.

How can Drupal pick it up? It is the field on admin pages.