A Bug with Darius' Q?

Recently, while spectating a friend's match, I noticed that while playing Darius on multiple occasions his Q ability did 0 damage. I thought this was a Spectating bug until he messaged me that Darius' Q did no damage in multiple cases to Champions, even when then blade did hit the target champion. His Q lifesteal did actively consistently work however.

I'm a tad confused about the whole thing and if this is a real bug with his Q.