The first thing that jumps out at me is you have the AppleScript appending it's value to a variable...which probably doesn’t matter since it's a local variable, but try switching that to save to variable and try it again.
Also, values returned from AppleScripts sometimes appear to be "true" or "false" or whatever your naming convention is you use in the AppleScript itself, but I have seen enough examples of where there's an extra space or something in there that the Keyboard Maestro If action does not like the
ìs condtiion; try switching that to
contains and try that too.
One other thing to try is changing the key
down to key
pressed as sometimes if the key is down and up again very quickly (like usually happens when typing) Keyboard Maestro might not catch that.
EDIT: Also... it's pretty early morning in the US where a fair amount of the forum resides so be patient and i'm sure more will chime in soon once everybody gets up and moving.