With your current product and discount settings, I am unable to replicate the problem you reported; that Discount Code is now taking half off the price of that test item, both in the cart and during checkout.
If you're still seeing the problem, it may be related to changing your discount and/or product settings while testing the cart with the same discount and/or product already in it. In this case, you can test from a clean slate by clearing your browser cookies, or just use your browser's private/incognito window to test.