I am also experiencing something similar:
Taxes are entered inclusive.
Selling to two countries.
Only one country has a tax rate.
It correctly includes or omits the taxable amount when switching addresses at checkout.
However using a coupon causes it to apply the tax on the non-taxable country. I suspect it is because the coupon grants free shipping, but the non-taxable country does not have a free shipping option (intentionally).
Thanks.
