Morde and Sett Ult bug or intentional?

https://outplayed.tv/media/vg8de/lol-mordekaiser-death

Ok I'm taking a break from posting stuff about old Aatrox. Just so Morde players are aware, if you are in the process of ulting someone, you may have a chance of being permanently suppressed by Sett during your ult. I'm not sure if the suppression carries out of the realm.