You've likely read in numerous places to use HUI mode with KeyLab and Cubase, but I've had no issues (well, maybe I had to do something for click long ago) with "Cubase" mode in MIDI Control Centre. Just for fun, I tried your configuration in Cubase 11 and 12, and I see the regular MIDI panning message as well (B0 10 7F, which is MIDI Channel 1, PAN, full right) when I use HUI mode. That would drive me nuts!
Maybe give the following a shot: set your MKII to "Cubase" mode in MIDI Control Centre, then under studio setup in Cubase 12, add a "Mackie Control" device, then ensure the Input and Output are connected to the MKII. You should be able to fully control Cubase from the MKII without the panning message appearing. I honestly don't know why that particular control message is being sent, but given the number of parameters and APIs Arturia has to address, it may be just a bug in the latest MKII firmware. I haven't tried this under Cubase 12, but it's been my setup on 11 (I'll give it a shot later on today).