This is a known, but confusing, limitation with Luminate. A recurring gift is viewed as one "gift" composed of multiple transactions, so when you go to refund part of a recurring gift it's paying attention to the total gift amount, not the amounts of individual installments. So by design, it is not possible to refund a portion of each installment, only to refund a portion of the whole gift, which will always be taken out of the last transaction for that gift.