The first hit of sonic Uair doesn't have rumble (most of the time?)

[OneTrueJay]

If you hit at the very startup of the move the first hit doesn't have rumble, only the launcher. But sometimes the initial linking hit DOES have rumble, I think it's when you hit after the very initial startup but before the proper launcher? I've only managed to have it happen 2-3 times in about 10 minutes of trying, so I'm really unsure what's causing the rare instances of rumble that do occur.