Coupon don't work as it should in Membership Plugin

Hi, the Plugin Membership is buggy, especially the coupon function. I create 2 different coupons for 2 different prices, each coupon shall discount 100%. ! coupon cost much lesser like the other. Problem is, i can use the cheaper Coupon also for the expensive subscripion, even when i select at the add coupon page the correct subscription, it work for all subscription, not only for the selected as it should!