@adavies The Inbound Shipment is a weird record If I remember correctly and you copy the uniquekey from the Inbound Shipment and you search for that line unique key in the PO you will not find it because is uniquekey in Inbound Shipment is not the same as uniquekey at line level of the PO.

The inbound shipment does use the same lineuniquekeys as the PO lines, but if there's multiple of the same item it seems to randomly select one of them to use as a dropdown option

Inbound shipment is really weird record, so much potential though