Have you set any of the controller knobs or faders on the MPK to CC#10?
I have an MPK (1st generation) too and some of the controllers on mine send out data too (once in a while).
If your device is still within warranty I would send it back.
If it isn’t, you could try to clean the physical pots and faders yourself with some compressed air (if you aren’t scared of opening the MPK, that is.)