Possible Garen Bug or Fix Needed

I m a Singed Top vs A Garen Top...I took Grasp of the Undying the Resolve Tree. Grasp of the Undying is proced by and Auto Attack..yet Garens Silence prevented the proc....for the full duration of silence.....by the time could AA and use it...his silence was off CD.it makes having the mastery useless and an AA shouldn't be silenced....This occured in multiple games.Thanks Guys,,,Merry Christmas.