Ahhh, of course it does. The filter is looking for any tasks that have an actual planned completion date in the past 4 weeks. Therefore it will not show incomplete tasks (because they have no completion date).
Ideally, we would have added another filter rule as an OR statement, to also show tasks that are not complete. But unfortunately, it looks like this isn't possible. Filters in the Workload balancer are applied as AND statements (i.e, when all filter conditions are true). You cannot add OR statement to filter rules in the Workload balancer.