add_filter('woocommerce_get_catalog_ordering_args', 'custom_woocommerce_get_catalog_ordering_args');

function custom_woocommerce_get_catalog_ordering_args($args) {
    // Add this debug line
    error_log('Sorting filter is being called. Args: ' . print_r($args, true));
    
    $args['orderby'] = 'meta_value';
    $args['meta_key'] = '_stock_status';
    $args['order'] = 'DESC';
    return $args;
}