Rengar ults, I flash, still take damage when he didn't hit me? Nor did his Bola strike hit me either. 
melee attacks actually don't seem to have a maximum range; it's only the range that it can be executed from the target that is limited. once the attack begins, the target will take damage regardless of position. that's why walking/dashing away from melee champs won't cut off their attack.
so unfortunately this is not a bug, just a quirk that's part of the game
however, Fizz's Q was special-cased to actually be dodgeable via repositioning, so maybe Riot will have mercy on ADCs with this particular ability. Maybe.
When he jumps on you, it's just an auto attack. Like all auto's, you can't miss it.
