Purchase Order Anomaly

I have a client who has reported an interesting anomaly with purchase receiving. This is version 17 (so it's possible that this has been addressed in a later version) but rather than upgrade I was hoping that someone here might have some clues about how we might address this situation.

The context is this: A purchase order has two line items for the same product. One line is stock to be sold, the other line item is stock for the shelves. Let's say that this product was booked on the P/O @ $5.00 each. When the receipt arrives we realize that the cost for the product is really only $2.00. When booking the receiver (via POST.APPO), the invoicing posting calculates the difference ($3.00) and attempts to adjust the cost on WHSE.STAT by a version of this amount. After the first line with this item is processed the cost on the WHSE.STAT (for future orders) is $2.00. So far, so good. But now we have a second line item for this same product showing an original cost of $5.00 and an adjusted cost of $2.00, with an adjustment amount again of $3.00. When this line gets booked, the $2.00 cost on WHSE.STAT is adjusted by $-3.00, resulting in a new cost of $-1.00 for future orders.

Anyone else see this? Anyone have a resolution they might like to share?

Followup: I checked the source for PROGS POST.APPO in a V19 install and the same logic exists there, so it would appear the problem still exists in the current version.