I can almost guarantee this is character specific (and possibly even stage). Many cross up J3's work differently against small and high hitboxes (i.e. actually GETTING a crossup on a large character like Doomsday is tough.)
But I suspect this wasn't tested thoroughly, not trying to step on your toes or anything. I remember playing with pretty much the exact same thing a while ago but discarded it because while it worked against the computer that was just standing, actually HOLDING BACK negated any mixup and would always hit in front.