Include woocommerce custom field value in front-end search result

You have to modify WooCommerce search query.

1. Hook into this action: do_action( 'woocommerce_product_query', $query, $instance );
2. Detect if it is a search query
3. Add your _subtitle meta

Something like this (not tested):

if ( $query->is_search ) {
    $query->set('meta_query', array(
        array(
            'key' => '_subtitle',
            'value' => $query->query_vars['s'],
            'compare' => 'LIKE'
        )
    ));
};