MRP and Inventory Reservations MD04

I have been digging on inventory reservations and have found nothing to indicate the the MRP element OrdRes is "decremented" by partial issues of inventory on process orders PI01 type. Were seeing the required quantity stay the same until final issue flag is set (or entire reserved qty issued) and final confirmation/clear reservations set on COR6N. Is this standard SAP functionality or is there configuration I've missed somewhere?

Tim