When the seller item price minus marketplace discounts has a negative value, it can't be applied to the item (they can't have a negative price), so it'll be considered as zero and will mess with other calculations. It happens when the marketplace is using the seller price override feature, generating higher values than the own seller account, and then combining with a promotion.

• In the marketplace, configure to overwrite the seller's price and define a higher price;
• Create a promotion in the marketplace with higher value than seller's price;
• Place an order;
• You will see the difference in the order's value between marketplace and seller.

Define the seller price equal or higher than the marketplace discounts value.