Hi,
this is unfortunately known a Flashplayer related limitation.
Internally it was necessary to draw the border on top of the background - otherwise there were holes and wrong overlappings between the background and the border (especially when using rounded edges).
And by drawing the alpha-transparent-border on the background the colors are mixing.
In the html5 version there wouldn't normally be such limitation, but there I'm actually simulating the Flash behavior to make Flash and HTML5 looking and working the same.
Best regards,
Klaus