Hi,
Could you please try to add a MIDI Monitor (in the best case some MIDI Monitor out of Cubase – like MIDI Monitor on Mac or MIDI-OX on Windows)…
Please double-check if your MIDI hardware controller doesn’t send any MIDI data continuously. The position of the slider/encoder could be at some “in-between” state (between 2 values), so it could send both (or one) values continuously. Just check, if there is any MIDI traffic by any chance.
Thank you!