Stamp doesn't work with FlxBitmapText
I tested this a few times and it seems that stamping a FlxBitmapText onto a FlxSprite doesn't work. It just crashes the game. FlxText works fine by the way.
FlxBitmapText isn't exactly the best thing in HaxeFlixel. Even the alpha's bugged.
Speaking of bugs, your problem probably is, and you could try inquiring in GitHub.