Workflow not able to get approver for AOR with scope business unit only

Summary: We have assigned the AOR's to employees with Business Unit as the scope in TEST pod. We thought of testing this out because maintaining AOR by department has become a hassle. When we assign these AOR's as the approver for workflow it is not able to fetch the approver in resource list.

If I configure and assign the same AOR for the expected approver with Department as scope then workflow does fetch the approver.

The person who submitted the workflow and the expected approver both belong to same business unit. My question why is the workflow not able to fetch the approver with the scope "Business Unit" ? Is this possible to switch AOR scope to BU rather than department ? We use these AOR's mostly for workflow approvals.