Odd behaviour with single modifier shortcut

I have an application shortcut for xfce4-popup-whiskermenu bound to 'Super L' (to mimic the behaviour on Windows with the start menu). However it doesn't seem to interact well with the default key binding for switching IME methods (Super + Space). Pressing the Super+Space causes xfce4-popup-whiskermenu to run, but the IME doesn't change (I have it bound to Control+Alt+Space for the moment, which does work).

If you have Super+Space assigned to another shortcut, both actions are triggered.

I think that if a key sequence triggers a shortcut, any modifiers should be ignored when they're released. It seems unlikely that you'd want the act of releasing the keys of one shortcut to trigger a different one.