Gravity blocks not falling when pushed by pistons

When bud-powering a piston wich is pushing many gravity blocks (eg. sand), those blocks won't fall down until the piston makes a short pause. Click for demonstration here or here. Like seen in the video, the piston gets bud-powered by the biston beneath it. Also: The bug only works if the pushed blocks have a block underneath them after pushing. Then no blocks are needed anymore, because they hover and get pushed further until they fall together.