Discount coupon applies on total amount

We created a coupon that activates automatically when a customer buys 2 or more products from a speciifc category.
It gives a discount of 20%.

When a customers buys 1 product from another (non-included) category and 1 from the included category, the coupon code does not work. That's how it should be.
When a customers buys 1 product from another (not-included) category and 2 from the included category, the coupon code is activated. That's ok, but here comes the problem. The discount is applied to the total amount, so to all the products also the one frome the non-included category. And that's not what we want.

On screenshot coupon1.png u can see 3 products. The third 'slip-on geel / cienta: 30" is not in the category ('stapelkorting') from the coupon. So the discount should be 20% from €110 + € 130, in total that would be € 48 and not € 53 (20% from € 265)


-- url of the page with the problem -- : www.theodoor.be/index.php/webshop
-- HikaShop version -- : 3.1.1
-- Joomla version -- : 3.6.5
-- Browser(s) name and version -- : Safari 10.1.1

It's just a single category (stapelkorting) without subcategories and specially made for this discount.
Because I had to exclude some brands and the coupon system doesn't work with the brands.
So I created a new category and placed all the products (from brands that should be discounted) through the mass-action-system in this new category.

I just send all the info.

Here are some more screenshots of the problem ...

Ok

Can you please inform me about the progress in this case? It's a week old and no new info

Perfect! Thanks!