This thing is an old bug, the game is supposed to render the preview bit on a black background with the block on it, but it usually ends up rendering just the black bit and you only see a few random colored pixels of the block on it. It's been like that for ages, on and off depending on the build of the game.
The best workaround for this is to reset the brush size sliders to one, then rotate the block to the desired orientation and then set the brush size sliders again to desired size. As far as I know the block preview thing on the actual build cursor has always worked right.