Ok, so adding some more specificity.
I was having that issue in one project. I opened a new empty project and tried the same thing on a single region.
In the new project, it did allow me to select notes, but I was able to get the behavior to reproduce after deleting a note.
So, let's say we enter 4 quarter notes. Now, I want a rest on beat 3. Command+Option+K to close Step Input, left arrow to select beat 3, and delete.
Now press left and right arrow. The score editor is still in Key Focus, but you can't select any notes, even after changing key focus back and forth.