After upgrading (OTA) my KA2 from 4.8.11073, I did experienced a strange phenomenon: in the home screen the menu-bar (indicating the menu, "home", background lighting, wifi, etc.) was completely gone and remained gone.
It did reappear when opening a book, only to be absent when returning to the home screen.
I did a device off/device on cycle and that seems to have cured it.