On the purchase order, the quantity invoiced is updated when a purchase invoice is processed using the Get Receipt Lines. However when navigating from the purchase order (Navigate > Invoices) to the invoices, the Posted Purchase Invoices list page is blank. There is no easy way to see which invoices were processed against the purchase order. You have to investigate via the purchase invoice find the receipt line, look up the posted receipt to determine which PO it belonged to rather than being able to go from the purchase order to the list of invoices.

The posted purchase invoice list should show the invoices processed against the purchase order via the navigate option.