Hello,
You say that you have perform an update, we don't know from which HikaShop version you have start the process, but there are a good chance that while this update you have the new way to process tax with Discount & Coupons.
And so I guess that you have set your configuration to reach your desired result, but as the management have change to be more consistent.
I know it can be a bit confusing but by the past and with other configuration reach the required configuration can be impossible.
Now, I analyze more your needs and your configuration screenshot from your first post, and on the other hand, you'right I make a mistake, you don't want to define in Coupon the Product tax category, leave it like you stetted it.
BUT you want to move in Main HikaShop configuration, and in Tax part, the Apply coupons option to After Taxes.
Can you process some test and confirm my idea, please ?
Regards