[GAMEPLAY] Sometimes rek'sai ult misses??

I've been messing around trying out the new rek'sai in ranked and some practice tool games and I've started to notice something that's happened multiple times... sometimes whenever I ult it will absolutely no damage and the ult was basically just to get me closer instead of execute. is this a bug orrrrr? Also I haven't been able to re-create it because it seems to only happen randomly.