OK, reinstalling doesn't make any difference.
I also upgraded to
this nightly koreader.
What happens is that the koreader screen is not erased when I exit, but the KUAL menu is still usable. Touching a KUAL menu rectangle makes it appear and either quitting KUAL or launching another app cleans up the screen.
Oh well, I can put up with this slight annoyance. Bettter (custom) dictionary, epub, pdf, djvu is worth it.