Xin Zhao VS Garen

I went Xin Zhao the other day and I noticed something odd and basically uncovered. I would use my q before going in.. but garens q made mine not work? From what I understand its already done and cast.. It's a buff.. like red or baron... Iverns or morgs shield... It goes on the buff bar and empowers your weapon... Garens q doesnt silence red buff.. doesnt take away morgs shield.. so why does it render xin's q useless? (The animation still runs.. the timer still goes down.. It just wont activate the on hit effect or the knock up) Is this a bug in the code? Maybe its coded like a channeled summoner spell?? IdK but from what i understand this shouldnt happen.