When running a receipting process, two values that are included by default are the PREVIOUSRECEIPTNUMBER and ORIGINALRECEIPTNUMBER. They are only populated when a receipt has already been generated for a revenue record, and then the revenue record was changed and the revenue needs to be re-receipted. When these fields are populated, the value for the receipt stack is appended to the original and previous receipt numbers, even though the receipt stack name is outputted in a separate column. This only occurs in version 2.6 and higher. In previous versions, the previous and original receipt numbers were correctly exported without the stack code appended to the number. This occurs for these fields regardless of the output format that is chosen for the receipts.
Download and install the latest patch, which contains all fixes from previous patches. If you are running an older version, download and install the latest version and then the patch.