Theme suggestion template is not being picked up
After you add your theme suggestion:
function discount_theme_suggestions_field_multiple_value_form_alter(array &amp;$suggestions, array $variables) {
    if ($variables['element']['#field_name'] == 'user_discount'){
      $suggestions[] = 'field_user_discount';
    }
}

You need to tell Drupal in this hook you should pick my template by implementing hook_theme like the following.
/**
 * Implements hook_theme().
 */
function MODULE_theme($existing, $type, $theme, $path) {
  return [
    'field_user_discount' =&gt; [
      'base hook' =&gt; 'field_multiple_value_form'
    ],
  ];
}

Now just put your template in you templates module folder clear cache and it will be picked.
Note: if you want to use a different name from the suggested one (field-user-discount) of your template like field-user-discount-something.html.twig just tell the hook_theme the name of template like:
 /**
 * Implements hook_theme().
 */
function MODULE_theme($existing, $type, $theme, $path) {
  return [
    'field_user_discount' =&gt; [
      'base hook' =&gt; 'field_multiple_value_form',
      'template' =&gt; 'field-user-discount-something'
    ],
  ];
}