1 Second after Lucio hit his ultimate (Armor was already applied) I head shot an Anna with 100% charge. I saw the hit marker and you could see it hit her in the replay. However, it took off 0 damage.
i’m a bit confused on what you mean, but if it was their lucio who used his ult 1 second prior, then ana would not take any damage at all from a full rail crit, or most damage from any attack for that matter. lucio’s ultimate gives himself and his teammates affected 750 overhealth that decays at a rate of 125 per second, and this also decays in combination with any damage dealt during this time.
so as soon as the ultimate landed, ana would have had 200 + 750 (950) health. one second later, when you landed your rail, she would have been at 200 + 625 (825) health; this would then be reduce by 190 because of your full rail crit. so, 825 - 190 is 635 health.
the only time i have died almost immediately after ult as lucio is when standing too close to a dva bomb ult. dva’s ult does 1000 damage maximum, which kills a 200 + 750 (950) health hero.