How does scale9Grid work?

In my project, I have a scale9 field for both my choices and buttons, but I am confused as to why the boundaries are not being retained when the application is resized and stretched out:

The result becomes pixelated as they be stretched, which I’m assuming is not how scaleGrid9 behaves (normally).

The code to apply scale9Grid goes like this:

var scale9:Dynamic = _convoData.choices.scale9;
if (scale9 != null)
      _choices.bmpScale9 = new Rectangle(scale9[0], scale9[1], scale9[2], scale9[3]);

The bmpScale9 is a property that sets and gets the underlying Bitmap’s scale9Grid value.

What this strangely results in is the above, using the latest version of OpenFL and Lime. Is this expected on Bitmap images or is this actually a bug? And if not, what am I doing wrong? This is the first time I’m using scale9Grid.

I’ve just checked the documentation on the API, and I’ve come across this:

All fills(including bitmaps, video, and gradients) are stretched to fit their shapes.

So Bitmap’s are stretched, even if scale9Grid is set, if I’m not mistaken.