Admin search not working for any type of post

you can try writing something that appends to your query var when the search is initiated something as such as:

    function gt_search_filter($query) {
      global $wp_query;
       if ($query->is_search)
        if($_REQUEST['s']){
           $wp_query->query_vars['s'] = $_REQUEST['s'];
        }
      return $query;
    }
    add_filter('pre_get_posts','gt_search_filter');

this worked for me but remember this is quite insecure and not advised what other thing you can do is adding more layers of filterations and everything before you finally give it to the query string.