I've just run into a weird bug on my Gentoo box. After some recent update the grave key stopped working. It did produce tildes when pressed with shift, but without a modifier it produced no output. Analysis with xev showed that the key was recognized as Zenkaku_Hankaku. Some more research revealed that it was due to this line in /usr/share/X11/xkb/keycodes/evdev and /usr/share/X11/xkb/keycodes/xfree86:

alias <HZTG> = <TLDE>;

Posting this just in case somebody else is struggling with the same problem.