I believe it only shows traders within 40 Ly of your current location, and only in stations you have already visited. Which makes the filter almost useless in many situations.
You should still be able to see the "material trader" station service if you check details for a station in system map - that works for any system with available exploration data, regardless of distance. There is the issue though that in combined economy types like industrial / extraction it is unclear which material types the trader deals in.
