Keybindings using xmodmap-redefined mod5 key not working

I have this line in my fvwmrc file: Key M A 5 Move - this means that a mod5 key + M should trigger the Move action, right? (Back when I had it as Key M A 4 Move it worked fine with the Windows key, which by default is Super_L which is a mod4 key, if I understand everything right.)

Today I used xmodmap to redefine my Windows key as Mode_switch, using xmodmap -e "keycode 133 = Mode_switch NoSymbol Mode_switch" - this looks like it worked, according to xev output.

The xmodmap command says that Mode_switch is a mod5 key: mod5 ISO_Level3_Shift (0x5c), Mode_switch (0xcb). So if I understand everything correctly, my Windows key, which is now Mode_switch, should count as a mod5 key, and therefore Windows+M should trigger my Key M A 5 Move keybinding - but it doesn’t. I’m not sure which part of how this works I’m wrong about…