I think I have an idea of what's happening here. There should be a short buffer period where pressing one button for the super slightly before the other button will still result in Velvet doing the super. You don't have to push both buttons at exactly the same time to get the super.
I'm thinking that the way Velvet's icicle works is that, once the quarter circle has been completed, Velvet plays out a pre-scripted animation sequence, and simultaneously tells the game to create an icicle, and give the icicle that same command. Because the icicle is a projectile, it's sequence is handled separately from Velvet's, so it doesn't need her to still be doing anything to do its thing. This makes it look like the icicle is given the order to do its thing almost at the same time as Velvet.
That's my guess as to what's happening: The game sees the quarter circle, sees a single button press, goes into "B" Icicle startup, reaches the frame where it tells the icicle to do its thing, then the game sees a second button press, goes "Oh crap, they actually wanted to do a super! Stop this move and start the super!", Velvet drops icicle startup and goes right into her super's startup, and the icicle simply doesn't realize that Velvet has started doing something else, because the icicle has already been told to start its script, and the icicle doesn't care what Velvet is doing.
I don't know for sure if I'm right about what's happening, and I don't know if this is really a "bug", but it definitely feels like unintended behavior. Nice catch.
