Customer should not be allowed to change the quantity after a coupon is applied to the cart. The application of coupon code may sound very simple but it’s actually not that simple because there are so many ways the customer can scam the system when you open the discount option. From a shop admin point of view you may not think about this much but some customers will try various dodgy things. Anyway, I have updated the cart so now it will show an warning when a customer tries to change item quantity after a coupon code has been applied to the cart.