In case you guys didn't know, if Kha'zix activates yommuu's while ulting he is made visible again. I saw a post on this a while ago, but Riot hasn't done anything about it. I think it's a bug (hehe) because Rengar can use Yommuu's while ulting with no problem. Kha'zix should be able to use yommuu's while ulting and I think a small change like this would be really nice :)
The easiest way to test to see if this is a bug or not is to get yomuus on teemo or eve and see if it breaks their stealths. I thought rengar's was special where it only broke on leap, but I'm not sure.
Edit: Rengar activates his predatory instincts, gaining stealth after 1 second of not taking damage or 3 seconds, and True Sight icon true sight of all nearby enemy champions for 7 / 12 seconds or until Rengar damages an enemy with one of his abilities or basic attacks.
In other words, his is exclusive to damage for breaking stealth
This post may fit better in "Report a Bug"
As a 
main, I'd like to see this issue fixed. Heck, I didn't even know this was an event that could happen.
