When having opened some built-in applications, like "Settings", pressing the F12 key does nothing.
I have it configured to open "Guake" terminal. This is made as a global shortcut in Guake itself. It works when I have focus on other application, for instance Chrome/FF browser etc.
I've checked shortcut settings, but F12 key is not bound to anything.
It did work on previous Ubuntu versions.
It seems that there is a workaround for it, but anyway it seems broken.