Mage double magic damage indicator

I think there is a bug where you cant see the double damage indicators on mage abilities. For example if you get an single target q off as Karthus the large purple crit number wont shot, in fact no damage numbers show at all just a draining health bar. I've tested this with Ryze E>Q and Anivia Q>E and the magic crit number wont show. Why is this like this??