This behaviour is by design and common amongst synthesizers that feature a voltage controlled analog filter. The filter has been designed to respond in a sensible manner to keytracking. This in turn determines how much you can open the filter by means of the filter frequency fader (FREQ). You can make use of the remaining headroom if you modulate the filter frequency with an envelope or an expression pedal via the modulation matrix, for example.
Moving the filter frequency fader to the highest position doesn't fully open the filter.
