Using shortcode with WP Bakery (Visual Composer) grid builder

Using the shortcode [favorite_button post_id="{{ post_data:ID }}"] within the grid builder I get the correct ID for the data attribute e.g. data-postid="123" but the data attribute for the favorite count is always data-favoritecount="0" instead of the correct number of favorites, and shows like this in the grid:
<span class="simplefavorite-button-count">0</span>
WP Bakery support pointed me to using a custom variable like on this page but I still can’t get it to work for me.

Using the shortcode on a single page works fine.

Any help gratefully received, thanks.