We have an "order update" web hook setup but also get the same results when just pulling the order using the Admin API - when the quantity of an order is reduced the discount amount is still based on the original order quantity. For example, there is an order for a quantity of 4 with 100% discount which equates to $35. The order is edited from the admin page to a quantity of 2. The resulting order still comes over with the $35 discount.
The below is from the documentation:
The following discount types are recalculated when an order is edited:
Percentage discounts that apply to the entire order — These discounts are recalculated when you add new items or change item quantities.
Discount codes for a fixed amount that apply to specific products or collections — These discounts are recalculated when you adjust the quantity of an item that already has the discount applied. This type of discount is recalculated only if the discount code can be used more than once per order.