Hi there,
I think the problem is that you check all the behaviors of the button in the Cornerstone itslef and that is not accurate. Cornerstone uses the iFrame technology and sometimes it can not behave exactly what the page will be if there are cases like yours which you used a 3rd party shortcode.
I suggest that you actually check the case in the front end of the website and there you will see that upon clicking on the Add to Cart button it will automatically redirect to the cart page and there is no message whatsoever. As that is the actual behavior of the button there is no meaning to have a message actually.