This appears to be a bug as I can reproduce it on multiple development instances.
Steps to reproduce:
- Add 2 items to cart 
- Apply Promotional Code from Pricelist 
- Remove first item from cart 
- Try to remove the second 
When you click on the minus on the second the number changes to 0 but it does not empty the cart or change the price.
 
                        
Did you solve this problem? I also have problem with coupon in cart. thanks