Why does nunu stay hidden after he completes channeling and deals damage with his ultimate/

Solution 1:

The answer was simple:

Nunu's ultimate is AOE (Area of Effect), Hes channeling his ult' while hiding in the bush does not reveal his location to an enemy. Why?

The reason: He is launching a skill which is directly connected to his champion model. The slow effect of his ultimate when you walk into his ultimate's range while he is channeling will also not be visible to you.

About the video: Sion is channeling his Q while hiding in the bush which also doesn't reveal him since his Q is connected to his champion model.

You only see the champion and their channeling skill if you have vision of that bush.

Lux's skills are projectiles. You might know where Lux is when you see the projectile flying towards which direction, but you can't get to know Lux's location directly.

Only auto attacks against an enemy unit will directly reveal a champion who is standing in a bush.