I can confirm this. CC121 does not respond to the track selected in the Projects Window. It will only respond to whichever track is selected in the Mixer.
I am having issues with CC121 responding to track changes on cubase 9.5.3.
If I am in mix view the channel select does not move left or right if I press arrows on cc121. If I am in project view the cc121 channel select works going from track to track but no buttons respond to the highlighted track instead they respond to the track it is stuck on in the mix view.
Seems like some kind of sync issues. Would love any ideas or help.
Thanks
Todd