Currently, when using cashback promotions, the calculations are entirely performed in the moment the items are inserted in the cart.

Hence, any changes to the orders done after that are not considered. Consequently, orders that had removed items and only consider the application of cashback after an invoice, the main case for this issue, will still take into account the removed item's cashback even though it was removed.

1 - Create a 10% cashback promotion, restricted to only yourself (to avoid incorrect applications) and with the invoiced condition to apply the credits.

2 - Close a $100 order composed of 2 items: one that costs $80 and the other $20.

3 - The applied credits should be $10. Remove the $8 item from the order and invoice it.

4 - You will still be credited $10 as a cashback, despite the expected value now being $2, which happens because the promotion calculations are done during the checkout simulation, before the order is closed.

There is no workaround.