Without the second liquid above, the ice displaces a volume of water exactly equal to its own weight. After it melts, the ice becomes the same volume in water, which is why the volume remains constant.
However, the upper liquid layer provides some buoyancy, so less of the ice cube is in the water than if there were no liquid above. Therefore, the ice displaces less water than before but the ice itself contains the same amount of water as before. After it melts, there will be an overall increase in the volume and the water level rises.