function wpf_filter_queries_standard_for_search( $mode, $query ) {

    if ( 'any' == $query->get( 'post_type' ) ) {
        $mode = 'standard';
    }

    return $mode;

}

add_filter( 'wpf_query_filtering_mode', 'wpf_filter_queries_standard_for_search', 10, 2 );