Planner search drop-down not restricted to items within a plan

While querying for supplies and demands, the Planner search criteria lists all the planner codes that are available in the application. Expactation was that it should list only the planner codes that are assigned to the items within the supply plan which is open.

We are using a single planning instance with multiple sources catering to different business units. The above behavior floods the planner list with codes from all the source instances across business units and it becomes difficult for planners to locate their code easily.

Why is planner codes drop-down list not restricted based on the items that are being planned in a supply plan?