You have to either set a mask over the actual text or make the background transparent.
When you create a bitmap in Flexi, or most anywhere else for that matter, you get a rectangle that bounds the subject of the bitmap. That rectangle has each and every pixel defined so when Allen Skin puts shadow behind the bitmap that's where it is, behind the opaque rectangular bitmap.
You want, in this case, to have just the text be rendered into a bitmap. Unfortunately, there's no way to do that in Flexi, so you have to use a mask or a transparency. This will effectively yield just what you need for shadow.