Invoice Approval Workflow does not have User Key

This question pertains to Oracle EBS 12.2.9 Payables Invoice Approval Workflow

A question for experts in Invoice Approvals / Workflow: We are implementing Invoice Approvals using AME and are finding that the User Key is not being populated in the Workflow Approvals. Typically, i.e. with Approval Workflows, the User Key is populated with the transaction number or some handle to assist in searching the notifications that may require admin handling. In this case one would expect the Invoice Number as a searchable value. However, I do not see that. It has no value at all. Is there a setup governing this or is there a way to have this field populated?