Note:This can be fixed by adding '_quality="best";' now.
This is well-documented side effect. Details are describe in Flash technique support without "Demo".
When _root has more than 1 frames, the quality of bitmap turns poor. When we make tween in _root, or we add preloader, we will got more than 1 frames in _root.
This can not be improved by quality setting in library or export options. To improve this poor quality, there is only one way: make the movie a single-famed movie. Collect this tween into a movieClip and make _root one frame only. The quality will improved.
The first movie is bitmap tweened in the _root. The second is tweened movieClip in a single-framed movie.
download the zipped fla