jarvan 4th passive

Was playing a game earlier today against jarvan and it is my understanding that his passive says " first hit deals 10% of targets current health as bonus physical damage, which caps at 400 damage. this effect cannot occur more than once blah blah every few seconds decreasing with lv" however i was being 3/4 shot depending on if u count the flag standard damage (hard to distinguish it from the pull personally) by his pull, 1 auto then ult. I was playing kha zix and with 2400 and some change health and was taking 815 damage from his passive in the recap, his auto attacks being a separate damage source. now im not a math wizz but that doesnt add up even without the fact that it states it caps at 400 damage. no, it did not proc 2 times i can promise you i wasnt surviving the 6 seconds it says it takes before it can work on the same target again at max lv. not sure what the problem is but thats the bug and im reporting it