[GAMEPLAY] Zac doesn't proc his passive

I'll make it short and quick:

I was playing top with Zac against Riven; about 4 minutes in, i get low and try to kill her taking in account my passive. She somehow damages me and kills me without activating my passive at any time. -My passive did not go on cooldown (it was used the next time i got to 0 health) -It did not happen again during the game

I hope you could maybe fix this or figure out how it happened, Thanks in advance :3

(You guys heard me once, I hope you will again)