Mixtrack Pro 3 Mapping Shift + Jogwheel to Deck 3 Scratch causes crossfader stuttering bug?

I’m trying to map my mixtrack pro 3 so I can use it with 4 decks, by holding down the shift key on each side to control the extra decks (far from ideal, but the only workaround). The only thing I’m having a problem with is the jog wheels.

If I map the jog wheels to scratch decks 3 or 4, it works fine with no problems. However, if I map them using shift + jog wheels to scratch decks 3/4, I get a weird interaction with the crossfader. When I go to use shift + jogwheels, the crossfader stutters around jumping left to right really really fast. When I stop touching the wheels, the crossfader returns to where it should be.

Am I doing something obviously wrong here? I’ve checked through the midi mappings and the only thing set to control the crossfader is the crossfader on the controller itself.