Then it’s a mystery. Because I’m on the same Cubase and OS X version you are. And it’s definitely not working for me. I’ve tried multiple key combos too, just to make sure there weren’t conflicts.
I’ve quit Cubase and reloaded my project. The key command has stopped working once again. Clearly there is an issue with Cubase. But only this shortcut. Any other keys I’ve set up for other commands always work and have no problems.