Woocommerce single_product_summary hook not working

In your first code block try changing return $output; with echo $output; and that should work provided $output is not empty 🙂

If you are trying to append your custom field to the product summary, this can be very well achieved using woocommerce_short_description filter hook. Please see below code:

function add_subtitle_to_product() {
    global $post;
    if (get_field('subtitle', get_the_ID()))
        return $post->post_excerpt . ''.get_field('subtitle', get_the_ID()).'';
    else
        return $post->post_excerpt;
}
add_action( 'woocommerce_short_description', 'add_subtitle_to_product', 10, 2 );