This might be an as designed thing, but there's a few things going on when trying to use a filter along with the scheduling page. Some might be designed, some might be buggered, and some I probably haven't discovered.
1. When selecting a base airport AND a freeform text filter (i.e. another destination code), Add Routes doesn't show any routes (nor does refreshing). It does not seem to matter whether the search criteria is met or not. I would expect that ALL routes could be added to a filtered schedule. This was primarily discovered when doing item #2 in this report.
2. When selecting a base airport AND a freeform text filter (i.e. another destination code), removing a single route from a schedule that causes the plane to no longer meet the filter requirements, it disappears from the scheduling grid even if the "Search" button was not executed again. I would hope that all results would stay there until a search was re-executed as it would be useful when rearranging a 7 day schedule (i.e. search for criteria that the planes you want to work with meet, then add/drop/shuffle routes - especially helpful in prop to jet transitions).
While the above 2 are with a base airport and freeform text selected, I'd imagine that other filter combinations would yield the same results. There's probably also other odd scenarios that I haven't hit yet...