title: Which hook to save WooCommerce product meta in WooCommerce?
body: <p>I am trying to update a custom meta on Woocommerce product update event. I've read that I should use woocommerce_update_product rather than save_post but I cannot figure out why only save_post works in my case.</p>
  
  <p>Below code works</p>
  
  <pre><code>add_action( 'save_post', 'mp_sync_on_product_save', 20);
  function mp_sync_on_product_save( $product_id ) {
       update_post_meta($product_id, 'test_acf_product', "text");
  };
  </code></pre>
  
  <p>Below code does not work</p>
  
  <pre><code>add_action( 'woocommerce_update_product', 'mp_sync_on_product_save', 20);
  function mp_sync_on_product_save( $product_id ) {
      update_post_meta($product_id, 'test_acf_product', "text");
  };
  </code></pre>
  
  <p>I accidentally found that if I add <code>exit;</code> at the end as below, the above code works (breaks page but meta saved in DB)</p>
  
  <pre><code>add_action( 'woocommerce_update_product', 'mp_sync_on_product_save', 20);
  function mp_sync_on_product_save( $product_id ) {
      update_post_meta($product_id, 'test_acf_product', "text");
      exit;
  };
  </code></pre>
  
  <p>I can get away with save_post but I'd love to know why woocommerce_update_product won't work
  I'd appreciate if anyone could give me some hint.</p>
  
  <p>Thank you!</p>