DJM-V10-LF Headphone output "Mix" won't play any Master volume

I'm trying to use the Mix knob in the Headphone A section of the mixer to balance the mix between a selected channel (Cue button selected) and the Master volume, but all I can ever hear is the selected cued channel which just reduces in volume as I turn the know towards the "Master" end of the scale.There is complete silence when the knob is fully at the Master end of the scale, despite the fact the Master LEDs show sound playing (and I can, of course, hear it through my Master speakers)

Is this defective hardware, or is there something I've accidentally pressed that's caused this Headphone mix button to not behave as expected?