Hey,
Thanks for reporting the issues:
1. Could you please explain little bit more about the issue, as I am not getting your issue fully.
2. I checked on my local system, discount coupon is applied only on product price not on tax. If product price is 100rp and tax is 23% and 100% coupon is applied so customer has to pay 23 rp amount only.
Let me know if I am getting wrong
Thanks