Try Courier or Courier New instead of _typewriter.
Same thing, no change (except the font changes).
I am having the same issue, but only when embedding the font. If I simply use fontFamily="Courier" in my mxml, it monospaces correctly.
So found this ticket which led to the following solution.
In my stylesheet where I style the text field that uses the embedded font, I used these 2 styles and now everything looks good!