Hello,
Because your product already have a discount and your coupon is using the "override", it should just generate a coupon for the difference between the product discount and the final coupon discount.
That feature/algorithm was provided by a third party and was integrated into HikaShop some time ago.
Unfortunately, that code is not something we have a full knowledge of and it looks like there is an issue because your coupon is twice the value it should be.
Please understand that it will need some time for our team to reproduce, analyze and fix the issue ; and we cannot do that during the week end (we are human, we also need some rest !)
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.