Duplicate invoice not found when voucher style is different

We had two vouchers entered with the same invoice number and the duplicate invoice settings marked are; supplier ID, invoice ID and business unit.  The invoices were both entered in the same business unit, same invoice number and supplier ID and the duplicate was not found.  However, the difference we found is one was a regular voucher and the other was an adjustment voucher.  The adjustment voucher was entered first and then the regular.  We have tested in a non-production environment and found when we entered the adjustment voucher and then entered a regular voucher it allowed us to save