Wukong ult vs rengar jump

I main Wukong, and was playing vs Rengar one game. After dashing into the enemy team and ulting, i was very low so, while ulting, i ran out of the fight. The enemy Rengar was in a bush and jumped on me, while i was ulting, and killed me instantly. Im sorry, but a Rengar jumping into my ult should be INSTANTLY knocked up at the very edge of my ult and left in the dust. I feel as though this bug may have something to do with a delay or not properly registering the ult, because in a separate scenario i ulted and ran into an enemy ashe and she was not knocked up until she was about halfway into my ulti animation. Wukongs play revolves highly around his ult, since it has such a high impact on team fights. Anyone who has not entered the ult already should be immediately knocked up unless they have spell-shield/kayle ult ect.