this is not an error, but the normal behaviour of the ModyPoly. The midi-filter value is added to the existing filter setting.
The range of the midi filter control is smaller than the full range, because midi controllers only have a resolution of 128 steps and we wanted to counteract a steppy control of the filter.
It is theoretically possible to alter the range of the filter control. This however would require a modification of the ModyPoly hardware.