正式なコメント
Sorry to hear about that -- unfortunately it's not a problem with the knob itself, as the encoder part (DSX-1080) is the exact same one used in all of these products:
CDJ-2000
CDJ-2000NXS
CDJ-2000NXS2
CDJ-3000
CDJ-900
CDJ-TOUR1
DDJ-RR
DDJ-RZ
DJM-S11
DJM-S11SE
DJM-S9
DJS-1000
XDJ-1000
XDJ-RR
XDJ-RX
XDJ-RX2
XDJ-RX3
XDJ-XZ
And given that the issue seems to be very restricted on the XZ, it could be something as simple as a faulty resistor.
I regret that you'll need to have the unit serviced again, but I hope they're able to address it for you.