menu_firstchild_menu_link_alter() interferes with other modules

Below function removes $item['options']['unaltered_hidden'] if a menu item is not for FIRSTCHILD like https://www.drupal.org/project/menu_inject_block
However, it may be altered by another module. The line should be removed.

Reminder: the hook is only used when adding/changing a menu link and saving it in the database.

/**
 * Implementation of hook_menu_link_alter().
 */
function menu_firstchild_menu_link_alter(&$item, $menu) {
  if (@$item['module'] == 'menu') {
    if ($item['link_path'] == '<firstchild>') {
      $item['options']['alter'] = TRUE;
      $item['options']['unaltered_hidden'] = $item['hidden'];
    }
    else {
      unset($item['options']['unaltered_hidden']);   // <-- This should be removed.
    }
  }
}