Well, Behringer states it will work with Cubase in the comment for the video I mentioned earlier in this thread.
Every Mackie Control thing I have tried with Cubase worked, whether with the BCF or a korg nanocontrol, or whatever thing I cobbled together. Maybe take a day off from trying to set it up and try again.
Also, try disabling Cubase prefs (search for Cubase Safe Start) in case this is caused by prefs corruption, but that’s not so likely these days.
edit- Raino points out something important. You need to be very detail oriented when troubleshooting. Difference between midiin (xtouch), or Cubase and Cubasis, etc.
My money is on the side of you getting this working!