Miss fortune immune to one of Ashe's AA, bug or cheat?

             As you can see, i flashed under the turret in order to kill  ![MissFortune](https://ddragon.leagueoflegends.com/cdn/16.11.1/img/champion/MissFortune.png) , but she seems to have been completely immune to my AA, which would have definitely killed her due to her really low health. I know that as soon as I flashed I was guaranteed to die too, but at least I would have gotten the kill, which whouldn't have made it so easy for MF and Leona to win the laning phase and, in the long run, the game.
             I would love to hear your opinion about this, please tell me: do you think this was a bug / some OP kind of script?