Lux Ult/Hitbox miss or Khazix Invulnerability?

I'm not sure what happened here to be honest. Khazix didn't have Death's Dance and his unit was on top of my ult. What gives? https://www.youtube.com/watch?v=N7VOruevtVM&feature=youtu.be

I've killed him after that with my ult again when he overstayed in our jungle. But that one instance he should have been hit at least. https://plays.tv/video/59cf7e5ddeb8478c74/killing-khazix