Note the PO should be copied before adding it to a bill.
Using an example of 3 lines ordered, 2 delivered, and invoiced, and needing to part receive this, is how to do the workaround.
1. Open original purchase order.
2. At bottom of the screen click more and copy. This should open a new order with exactly same info as first order.
3. Delete the two lines that are received, save and close the order. That “duplicated order” then becomes the order for the undelivered items.
4. When you close it, it should take you back to the original order, delete the line that was not received, save and close.
5. Match invoice to adjusted original order.
This can also be processed for those purchase orders that need the quantity adjusted. Instead of deleting the line you would just adjust quantities on the “duplicate order” and on the existing order to reflect how many were delivered.