For a drop ship sales Order ,any attempt to do the partial receipt against the corresponding PO (created for drop ship sales order line) will result in drop ship order line split.
After the drop ship sales order line got split , PO shipment will point to the newly created open line (created after the split ) and shipped qty set to NULL , their will be no relation ship between PO Shipment and the Original sales order line.
On the other hand , if we received in FULL against the PO ( created for a drop ship Sales Order line) their will not be any SO line split , and Order line will progress to Shipped and finally close. PO Shipment will point to the same (original) sales order line , with Shipped Qty same as Sales order Ordered Qty.
If User try ti cancel the drop ship sales order , it will cancel the SO Order line , corresponding requisition and Closed the PO.