I think the behaviour of this feature is not very safe.
When you add a set of products to cart in doscount and then increase quantity of them, all each product is in discount.
For example:
1.
Cart:
Prod1 (quantity=1) discounted
Prod2 (quantity=1) discounted
2.
Customer increases quantity of Prod2 by +1
3.
Cart:
Prod1 (quantity=1) discounted
Prod2 (quantity=2) discounted each item of this product
I think that only each set should be discaounted or discount should be applied in sets like
all in quantity = 1, all in quantity =2 ...