The info record update does not update any price conditions from the PO towards the info record, it only adds the PO number to the info record.
When you create the next PO and you still do not have any conditions in the inforecord, then SAP reads this last PO number and fetches the conditions from this last PO.
Can you post a screen shot from your pricing schema?
I remember a SAP note about a wrong sub-total indicator which caused that the price increased each time the user hit enter.