Merge array recursive

Closes #3447050

menu_link_attributes.module

@@ -9,6 +9,7 @@ use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Url;
 use Drupal\menu_link_content\MenuLinkContentInterface;
+use Drupal\Component\Utility\NestedArray;
 
 /**
  * Implements hook_form_BASE_FORM_ID_alter().
@@ -187,8 +188,9 @@ function menu_link_attributes_menu_link_content_form_entity_builder($entity_type
     $menu_link_options = $original->get('link')->first()->options;
   }
 
+  // Merge with existing options:
   $existing_options = $menu_link->link->first()->options;
-  $menu_link->link->first()->options = array_merge($existing_options, $menu_link_options);
+  $menu_link->link->first()->options = NestedArray::mergeDeep($existing_options, $menu_link_options);
 
   // Set for other languages.
   if ($menu_link->isDefaultTranslation() || !$menu_link->isDefaultTranslationAffectedOnly()) {