Advanced Product Quantity for WooCommerce

When in configuration settings “Show / Hide Quantity box on listing pages” is activated, the ajax add to cart functionality no longer works, instead on click the single product page is loaded. We would like to keep the ajax add to cart behaviour.

(I couldn’t find any support forum, support tickets, contact or something like this. For me this is not a feature, for me the current behaviour is a bug.
I didn’t buy the plugin myself. A client of mine bought it and I’m trying to troubleshoot his website against this.)