Coupon with fixed value bug?

[ConH]

In a webshop dedicated to our customers employees I am seeing this strange behaviour.
Every employee gets a (blank) coupon with value of 850 euro, for buying stuff for their home office.
When someone buys within budget the invoice looks absolutely neat. The goods are summed and an equal amount of the coupon gets deducted to reach € 0,00.



But if someone buys above budget, they have to pay for the surplus, and something strange happens:


The amounts in the Total column change to zero, but the totalisation below is correct. The maximum coupon discount is deducted and the leftover can be paid as usual. But since the products are "sold" as € 0,00 my reports are not correct.

Any idea why this happens? It is a structural problem, and happens only when people buy over budget, so it must have something to do with the implementation of coupons.

Presta version is 1.7.8.2
 

Edited January 5, 2022 by ConH (see edit history)