At present, when the Ignore List Price Decreases and Auto Price Update options are selected, product unit prices are the recalculated if a delivery item List Price is less than the existing List Price on the Product's Supplier tab.
This only applies to the supplier that the delivery relates to. If the current prices were calculated from a different supplier relationship, then the prices can still be recalculated.
To avoid this, price updates should be ignored if the calculated cost price is less than the exist cost price. i.e. if Auto Price Update is true and:
Ignore List Price Decreases is false, update unit prices
Ignore List Price Decreases is true, and:
the new calculated cost price is less than the existing unit price(s) cost price, don't update; otherwise