Purchase order item rate negative

Standard selling and Standard buying are set.

Create Sales order, the item price will auto match the standard selling price, which is correct.

than I created MR from SO:


created PO from MR, the price does not match the standard buying, and there is a discount amount: always equals to (standard selling - standard buying), in this case is 20-19 = 1. And the price rate is 19, SO THE FINAL is 19-1=18. If this kind of bug?