For this particular vendor, go to the Transaction screen within Vendor Inquiry. This will show a list of all 'current' transactions for that vendor, whether or not they have been completed. For Invoice 21-1, what is the 'Current Amount'? How about other transactions and their 'Current Amounts'? The Aged Overdue Receivables report will automatically suppress vendors with a balance of zero, so if vendors are printing on this report then they don't have a balance of zero. There is no option to include Historical transactions on this report, so this has no bearing on what you are running into here.

If a transaction has been set to have a Current Amount of zero (either by payment or adjustment) and cleared to history, then it will not affect nor appear on either the Open Payables or the Aged Overdue Receivables reports. You cannot apply any type of transaction (payment, credit note, debit note or adjustment) to a historical document because it has been completed.

If the problem here is that an invoice was completed because it was adjusted to zero rather than being paid, then everything was sent to History before correcting this mistake, then there is nothing you can do. If a transaction in Current needs to be corrected, then you can do something about that.

I see that Andrew has offered to look at your data. In the meantime you should be aware that both of these reports only use Current Transactions (ie not in History), so use the Detailed versions of these reports (without 'Applied Details' and compare to the Transaction list within Vendor Inquiry. You may find the answer to your queries in doing this comparison.