What is this empty gray region in the Key Editor?

The gray region is as-designed. There is nothing wrong.

You seem to have your own idea about how things work, which causes you to misinterpret what others tell you.

I can be snarky once in a while. It’s not so bad to be likened to Lucy from Peanuts, and it’s possible that someone else will say that I got that wrong.

If you want to report a bug use this method: How to report a bug in Cubase