Every time you change the bitmapData source for a Bitmap you also need to set the smoothing property on your Bitmap to true (if you want the bitmap to use smoothing).

var bitmap : Bitmap = new Bitmap();
bitmap.smoothing = true;
trace(bitmap.smoothing); // true
bitmap.bitmapData = new BitmapData(320, 240);
trace(bitmap.smoothing); // false

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