AME substitution rule not working for purchase requisition approvals

Hello everyone,

We are using AME substitution rule type to substitute an approver (HR person) with another one for the approval of some transaction types in EBS to manage "vacation rules".
It works fine with the transaction types that are currently set up in AME.

So we wanted to do the same for purchase requisition approvals.
(For some operational constraints, we do not want to use regular vacation rules to manage the delegation/transfer of approvals in PO module)
However, the system ignores the AME substitution rule and the approval is routed to the original approver of the requisition.

FYI, we are not using AME to manage purchase requisition approvals. Just standard approval using workflow and supervisor relationship ("Approval Transaction Type" field is not populated in the Setup > Purchasing > Document Types > Type = Purchase Requisition). So maybe that's why it's not working as expected?