OSN Rejections (Oracle 11.5.10)

I've noticed that occassionally during the Import validation process an invoice will reject back to the supplier for the following reason:

Invalid Invoice Line Number. I can clearly see the invoice line number the supplier sent via OSN, however, the notification that is sent back to the supplier indicates a different invoice number:

Example:

PO set up with one line. Invoice reference PO Line 1, Invoice Line 1. Validation process rejects the invoice back to the supplier

due to invalid Invoice line Number. Invoice Line Number 2.

We have seen this happen periodically since we rolled out iProcurement and OSN over a year ago.