[GNC] Invoice numbers within Customer Report

[Liz]

I find the invoice numbers do not appear consistently within the
Customer Report.
I can't say when it started, because only in the last few years have I
been using the Customer Reports consistently.
It does not matter if the invoice number is external to Gnucash or is
generated by Gnucash.

I've filed a bug
Bug 799386 - Invoice numbers do not always appear on customer report 

and welcome anyone else using Customer Reports to add any information
if they see the same behaviour.

Liz