Hi,
I agree with you, let me try to tell you why in my opinion and to prtopose a workaround in the end.
The Internal product receipt is stored in a different table, in the VendPackingSlipVersion table, and the field is called InternalPackingSlipId, so you can only see it if you go to the history button, just like the voucher (to which it is strictly related, see below):
In fact, if you Cancel the packing slip, both the Voucher and Internal Product receipt will be created new, because it is related to the posting of the same product receipt that was cancelled:
So that is why it cannot be shown in the Product receipt list, because is related to the History, so just like the Voucher you not have a way of seeing it on the Packing slip but only on the history, because is more related to the financial transaction.
If you notice the Internal Product receipt is a number sequence setup under Accounts payable parameters:
And is just a way for the system to recognize the unique product receipt (of course the "Real" number that matters is the one that is received from vendors). By the way, I usually activate the "Reuse number" so that I have the Voucher and the Internal product receipt matching:
Coming straight to your question, by using the same number for voucher and internal product receipt (the "Reuse" button above), you can refer to one of them in the financial voucher or in the transaction in the "Physical voucher" like shown below: