Workload Balancer - 'Unassigned Work' area to allow filtering by Issue/Request

In Workload Balancer (WLB), we need to filter down the displayed items on Issues/Requests only, using a specific value in a custom field.

This can be done easily in the 'Assigned Work' section, but in the "Unassigned Work" section, the option to filter on 'Issue' fields is not available.

I have had it confirmed this is by design but seems odd it is in one area and not the other.
(doc attached with screenshots)