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

I know the question title seems subjective, however the reasoning behind that is because its not an easy question for me to phrase right now. The example I will use is Sion, Lux, and Nami.

These champions have knock up, slow and knock up skill shot abilities that cc the target and deal damage.
Now if I were in the brush and cast these skills, after the target is damaged they gain vision of me if I am within range, right!? so in Lux’s case she would q e then kill you with ult, crowd controlling you with q so that you do not react to her e. In Nami’s case she would q and then and with the help of the adc chunk you. You see her and Lux where the damage is applied. I am not 100% certain about Nami as this is something I just recently thought of and I have not had to deal with a Nami in that scenario recently to make any comments about it. So I watched a stream where sneaky was q knocked up by Sion followed by an e that did not reveal Sion’s location through out the entire exchange.

If you watch it you will notice that after he landed he was momentary stunned and though he issued a move command. His champion did not move until after the stun duration. During the stun duration he is hit with an e you see the ability exit the brush but Sion’s location is not given before and after the e reaches sneaky’s Twitch.

Which leads me to believe that Crowd control can affect a champions vision. Taking that statement to consideration why is Nunu’s location not given while he slows a team with his ultimate and after he deals damage. His ultimate is a slow so why is it he stays hidden?