Screenshots of how you configured single level in approval flow type, selected an option from first list to find the user from application and an option from second list of users/user references based on the option you selected in first list will help us suggest you what could have gone wrong.
also what’s the clipboard value for participant party in that thread context? By any chance was the participant “admin”?
Below is the screenshot. The operator that is creating the case is being added as a work party through VOE (CurrentOperator DT) . pyWorkPage shows participant count as 1 and under workparty, I see all the details of the work party.