Lucian exhaust passive bug on minions [Gameplay]

Hey, so i was watching the latest gosu video and i noticed a bug with lucians passive when you are exhausted: https://youtu.be/M9vyC4E_Zss?t=4m35s You see that gosu gets exhausted by Sona and then tries to lasthit a caster minion, he has his double shot up and normally his autoattack would have killed the first minion so his passive would go onto the next minion, but because of the exhaust from sona Gosus autoattack doesnt kill the first minion, his passive still goes on to the next minion tho. It seems like thats a bug, or is this intended?