Buy X Get Y discount wrong calculation

Hello,

I’m interested to buy your pro plugin, but it seems there be a problem. I tested on your demo site, to offer 10% on a product when we buy another product. The rule applies well, but the calculation is wrong.
See the screenshot: https://ibb.co/8dGgVVq
The product has a 10% discount, but the “subtotal” column displays the difference ($15 – $13.50 = $1.50) and not $ 13.50.
Can you take a look?

Thank you

The page I need help with: [log in to see the link]