I've identified the two knobs (let's call them knob A and B) that control the volume and the panning on my keyboard. I never assigned them as such myself so no idea where that comes from. When I reset the panning/volume back to 0 using knob A and B, within a few seconds the panning jumps back to +43 again.
Ok, I've also just tested in again without the keyboard attached and the panning still jumps back to +43!! This is very weird.