In the short term, why not re-create a #13 to the value of the credit so that your new invoice is unaffected by any credit. You could leave the account as “suspense” so its highlighted in the system. Then when you have an opportunity you can explore further, even asking the customer for a copy of their #13 so you can amend the details of the re-created #13.
Generally, invoices don’t delete themselves but sometimes computers can do strange things.