(Patch 6.6, EUW-Server) Bug between Kalista Ultimate and Zac´s Jump

Hello dear Riot Team

I played Kalista in a ARAM game today and in my team there was a Zac. I gave him the spear that i can use my Ultimate (Fate´s Call) with him. And 2-3 times it happened that when Zac charged up his E (Elastic Slingshot) and i pressed my Ultimate, that he didn´t get untargetable and he is still on his place, while my ultimate goes on cooldown. But after the use of his ability or when he get interrupted by hard cc, than he will get affacted by my ability.