PO # doesn't show in PDF after editing an invoice.

PO # doesn't show in PDF after editing an invoice.

If I create an invoice, including a PO# at the time I create it, and export it as a PDF, the PO# appears as expected.    If I edit an invoice after creation, and add a PO#, the PO# will not appear in the exported PDF, despite appearing in the summary.   

Any hints?