Hi SynthGuy,
It is 100% a fault, usually the MG modulation is exclusively VCO, VCF or VCA.
The only way that a working P6 would have multiple MG destinations is by using the Mod Wheel to introduce some VCO Modulation.
I imagine the one I am having trouble with is the victim of battery leakage and even though it has got a cloned KLM board I expect that the original owner may have transplanted damaged components to it from the original board.
Good luck, there are some really kind and helpful guys on here willing to help out.
Jim