AP Special Handling Extension not working - Oracle 12.1.3

If a Suppllier has two or more invoices for processing and payment on the same day and one or more of the invoices has been flagged (Invoice Header DFF) as requiring special handling e.g. requiring a document attachment to the cheque. The invoice header special handling extension does not create separate and sorted special handling payments... i.e. the system creates a single cheque for both(mulitple) invoices.  When an invoice is flagged for special handling we would expect payment for that one invoice only.  Is there a special step we have to do to ensure this happens or is this a known