Issue VAT (BTW) when using Coupons higher than total price

-- HikaShop version -- : 6.5.2
-- Joomla version -- : 5.4.7.
-- Browser(s) name and version -- : Ecosia

Hi all,

I have discovered a VAT calculation issue in HikaShop that I can't seem to get fixed. I would appreciate it if you could check whether my analysis is correct and, if so, advise on the best solution or provide a fix.

The Issue
The issue occurs when using a coupon with a value higher than the remaining product subtotal or the exact same amount.
If this happens, HikaShop stops showing the VAT (BTW) that is being calculated. The VAT is still calculated, but not shown anymore.
In these examples, our webshop uses 9% reduced VAT for food products. Normally, the VAT calculation is correct.

For example:
Example 1 – correct behaviour - Screenshot 1
Products:
€83.88 (so this amount is including 9% VAT)
VAT:
€6.93
Coupon:
-€80.00
Final total:
€3.88
Displayed VAT:
€6.93
This is correct. The VAT is calculated as it should be: ((83,88 / 109) x 9) = 6,93.

However, when the coupon value becomes higher or exactly the same as the product subtotal, the VAT calculation in the checkout changes unexpectedly.

Example 2 – incorrect behaviour - Screenshot 2
Products:
€76,89 (it still works with the price of the products including VAT here, so the VAT is still used in a correct way by the system)
Coupon:
-€80.00
Final total:
€0.00
As you can see, it does not show VAT anymore.

It seems not to calculate once the total amount is €0,-. When there are shipping costs, it only calculates VAT over the shipping:

In the example below, we have shipping costs of €4.50 if someone buys for less than €75.00. Therefore I lowered the amount of groceries till below €75,00. Still only food products, so only 9% VAT.

Example 3: Shipping Costs - Incorrect behaviour - Screenshot 3
Products:
€69.90
Coupon:
-€80.00
Shipping:
€4.50
Final total:
€4.50
Displayed VAT:
€0.37

The VAT shown here is no VAT over the products. The VAT over the products AND shipping is supposed to be €6,14:
Product VAT: (69,90 / 109) * 9 = 5,77 
Shipping VAT: (4,50/109)* 9 = ,37
In total: 5,77 + 0,37 = €6,14.

However, the VAT shown here is only ,37. So it only shows the VAT of shipping costs.
But this is incorrect, because even though someone receives a discount via a coupon, he has still paid VAT over the products. So the correct VAT statement here should be €6,14.

Example 4: Discount = exactly product costs - Incorrect behaviour - Screenshot 4
Also when the discount voucher is exactly the same as the amount of products, it does not show VAT.
Products: 69,90
Discount: €69,90
Shipping:
€4.50
Final total:
€4.50
Displayed VAT:
€0.37

Observation & analyses
It appears that when the coupon reduces the taxable product amount below zero, HikaShop sets the product taxable amount to zero. This is not correct: even though somebody received a voucher with a certain value, VAT (or BTW) is still being paid.
The system does perfectly understand this when there is still an amount to be paid, even if it is €0,01. The VAT remains correct. But when the amount goes to €0,-, it does not show the VAT anymore.
This current situation means that the information shown to our customers is incorrect, but also that the VAT we have to pay to the government is not shown anymore in our report and therefore can not be calculated. This is a big issue for us.

Possible Solutions
Could you please look into this issue and let me know what the recommended solution would be? Since we can not use coupons right now, I would appreciate a fix or workaround as soon as possible.
If you need any more information from us, please let me know.

Regards, Sven