It’s caused by a new feature in C9 wherein flats are shown in the Key Editor according to the current chord on the chord track. You won’t see A# unless you activate a chord track and insert a chord whose scale contains A#.
It appears that Bb is the only note that is flatted by default.
The setting you mentioned does work here, but is for respelling Bb and B to the German B and H.
It will have more sense to see Bb only when you enable chord track with particular chord, and A# to remain as it was through 15+ years how I work with Cubase.
Not so fast. By chance I posted an issue report on this yesterday & was happy to see it was supposedly resolved today.
However my test Project, now saved as 9.0.10, shows the problem still exists. I did an update in the issues forum and am waiting its approval. But in my OP there I posted this screenshot of the problem and it looks exactly the same in 9.0.10.