Maokai passive bug vs fiora

I was playing a ranked game and got a gank from rengar, i was playing maokai vs fiora. but when i went in i got stunned by fiora w for no reason. after looking back the footage it was not my q not my e and not my w what stunned me of fiora w. the thing that stunned me was my auto attack with passive on. because of that i died and fiora ended up snoballing. but shoudnt this be a bugg???. when you read maokai passive it isnt something immobalising so it shoudnt end becomming a stun of fiora w. riot plz fix this for the incomming patch. btw i have video proof if you want to see it.