Hello Gideon
Technically, when a sales order is created, it should consume the PIR and the PIR quantity should be reduced by the sales order quantity.
If this is not happening, this is a problem on the PIR consumption. You should check the PIR consumption parameters on tab MRP3 of the material master and make sure that the PIR is within the consumption horizon.
I also wrote the following document with links for the demand management FAQ notes. Please read those notes carefully, as they provide the solution to the most frequent PIR consumption issues:
Demand Management: FAQ and consulting notes
BR
Caetano