iSupplier portal - withdraw or cancel invoices on the portal

We have implemented iSupplier portal and planning to utilize the AME approvals to monitor invoices that are submitted on the portal. The AME rules are enabled in a test environment and the ISP invoices are going through the approval process as intended. On the approval notifications there are a couple of options for the users to respond (Accept, Reject). The accept process is straightforward and working as intended. However when we test the reject process, the invoice status changes to rejected and on the portal the supplier cannot withdraw or cancel this invoice as these buttons are greyed out. Has