That is the very same issue I had, and was just about to post.
I assigned a voice command to press the \ (backlash key).
In Elite Dangerous I assigned this key to Landing Gear.
I was talking until I was blue in the face, and it would not respond.
If I press the "\" key on my keyboard in-game, it responds fine.
However, I alt-tabbed out of the game and opened Notepad.
I said my voice command for landing gear, and a "\" character was correctly typed into notepad.
Went back to the game and nothing.
I even tried this key in DX input and standard, but still nothing in-game.
I then changed the key being used in VA and in-game which worked fine.