[Gameplay] is thorn mail bugged?

so basically the youtube video has all the info. but to beak it down here is what happened. i was in a game as garen vs a quinn and noticed that while i was taking the tower she was taking damage from thorn mail. however i never got aggro from the tower. i began to wonder if i was seeing things or if this was a bug or something. so me and my friend tried a similar theory using garen and jinx where if i was just under tower and thorn mail was hitting her she would take damage and i would not take turret aggro. this turns out to be the case. So I'm wondering if this is a bug with the item because in theory it should be similar to the Sunfire Cape in passive aoe damage and make the turret aggro on me. but that is what I think, i may be wrong. just putting it out there in case something might need to be fixed. in the video, all we did was farm for a few minutes so i could buy thornmail, then proceeded to test it under tower using jinx with nothing but attack speed.