Do try executing the command without using push-to-talk (I.E. by clicking the button on the main window to manually turn listening on)
DCS allows arbitrary keys or joystick buttons to be assigned as ingame modifiers, so if your push-to-talk button/key happens to be one, that could interfere with normal input.
As mentioned, modifier keys cannot be prevented from passing through, so you'll also want to use a key (combination) that is not/does not include a modifier key (Ctrl, Shift, Alt, or Win) as your push-to-talk key.