XinZao ult Bug

soon as i cant get a physical example all i can do is write this in, when defending a turret i was near full health and (as ad/crits full build thresh top) xin zaos ult managed to one shot me, this is ither a bug with zin or where the recap is bugged because the ult only equaled 246 damage, along with another 250 damage being from yasuo. i had a max hp bar of 2400-2500 + shields. I am unsure wether this is a UI recap bug, xin bug or a horrendously massive error with champion ballance in 4.20 but this bug cost me a 1 hour game and i dont like it when this sort of tacky issue causes injustice