Let's show in images :
Here what we see :
And what we really get :
We can see indeed two comets in AoU meter, a little one and a bigger one. And after at least 100 comets thrown, I always got a double one. Which confirms the fact that rubis tells right -> You have 100% chance to get an additional comet, dealing less damage (or there would the fact that I would be clearly lucky, but I don't think it is the case

).
So yes, like me and surely a lot of people, it's only a graphic bug. We don't see the additional comet, but it is truely there