ACF does not fire on this hook save_post_product, it saves only on the save_post hook which happens after the post type specific hook you are using. This means that ACF has not saved any fields before you are trying to get the values of the field.
Your choices here are
1) use the acf/save_post hook https://www.advancedcustomfields.com/resources/acf-save_post/ priority > 10
2) access the submitted values directly in the $_POST[‘acf’] varialble, information on this is also given in the doc link above.
