They "should" be the same, but they seem different because of the visuals, and you might be using your abs better on your back because of the constant exhale.
Try closing your eyes, and make sure you aren't breaking streamline to look for the surface. The surface is still in the same place it was on the last lap, and the lap before that, and the lap before that.....
also try starting the dolphin kick on your back, then rotate to your stomach while kicking, trying to keep the same motion going the whole way.
Swimming Workout of the Day: Favourite Swim Sets: 2020 National Masters Champion - M50-54 - 50m Butterfly