Hi,
Are you sure that the option "add to cart legacy" has been turned off ?
On your listing, for that product, I get an add to cart button.
I would recommend to try renaming the folder templates/YOUR_TEMPLATE/html/com_hikashop/product/ when you turn off the "add to cart legacy" in order to deactivate the view overrides that you have for the products listing.
That way, you can check whether the issue you have with the new add to cart system comes from the customized views you have or if you also have the problem with the new views.
If you don't have the problem with that folder renamed, then you can put back the original name of the folder, and do the same for each file in it in order to identify the file causing the issue and then you can redo your customization on the new version of that view file via the menu Display>Views.