Coupon code not applied automatically

I'm using Stripe with Spark and I have created a coupon code there. Now if I open my register form with a query string domain.com/register?coupon=COUPON there is an proper information saying "The coupon 30% discount will be applied to your subscription!".

But then when user decides to subscribe after trial, in subscription form the Coupon field is empty and if user doesn't remember the code or forgets to fill it, no discount is applied.

Is this really the way this should work or is something broken in my installation?