I'am not a hundred percent sure, but Filter Search uses your whole library as source when you perform a search, not the currently active playlist. And I don't think that you can change this behavior. Instead you maybe want to take a look at Quick Search Toolbar. There you can choose, where to perform your search in - Database, Playlist, All Playlist etc.. Here are some further information about how to use it.
I use Quicksearch plugin as well. Excellent alternative. Searche in database by defautl, but in one click, it will search in current playlist.