Comment 3 for bug 1307897

Light-locker doesn't interact with the numpad at all (and in my opinion it shouldn't).

This is most likely a problem of VT Switching and X11 taking control over the Keyboard in the greeter. The basic problem being: how should the greeter know whether numlock was on/off during the session?
(A service like accountsservice would be an option, but I don't think it has support for numlock status. Also, currently xfce4-session doesn't save/restore the status of the numpad, so upon a reboot or a re-login you have to manually run a programm like xnumlock to set the status.)

You can verify that this is not a bug in light-locker by deactivating it in light-locker-settings and then trying to lock the session from the terminal with "dm-tool lock".