Is this an account assigned PO?
I have tested in my system for purhase to inventory, by changing the info record price after creating the PO. PO created on 6th adopted the price relevant for 6th, I maintained a price valid for 7th, then went back to the po to enter a confirmation on 7th,, The price is not getting changed to the 7th price of info record, unless i redetermine the prices.
i even changed pur group del date etc without any effect on the PO