When you create one PO with one line with quantity bigger then one and then create multiple Item arrival journals for different quantities you have the option to select all of them and generate product receipts. The issue is that the system will sum up the quantities and generate only one product receipt and the number will be reflecting the packing slip of the last item arrival journal. The most logical behavior will be to generate a product receipt for each Item arrival journal. This is how the system behaves in case you have different items in the PO lines tab and create Item arrival journal for each line.