I ask because your first screen shot shows a zero-value sales invoice, but nothing appears in the Customer Summary. You later say that adding a zero-value invoice causes the customer to appear on that report. These are apparent contradictions.
The zero-valued invoice appeared when I added the unpaid invoice for the starting balance. I don’t know why there three line representing the starting balance.
When I changed the invoice I added later to 1 USD the three “starting balance” invoice lines remain
I have this bug too. I had a check with custom reports like this and all the Customer and Suppliers with only starting balance are not there in the Summary Reports.