Why Pushback button is not hidden for Manual Settlement Approve Method?
Pushback button is not applicable for Manual Approval Method. This works with Approval Framework and is only available for second or subsequent approvers in a multiple-approver workflow path. Selecting this button will send the request back to the previous approver and hence this is not applicable for Manual Approval where only one user approves the transaction.
I would suggest to hide the Pushback button for Manual Approval Method.