Genuinely Curious What's Going On With Jax.

What am I missing with Jax's Ultra Damage Reduction lately? Like I haven't played in a while so did I miss a Patch where he is uber now? I was Miss Fortune With Ghostblade, BC, and Duskblade, with 23 DH stacks, even level with him and I hit him with a Q Double Tapped off a dead minion, normally that should be a death sentence but he somehow reduced 500 of my 700+ Damage down to 230. And he only had Giant's Belt, Trinity Force, and Warden's Mail. Like, Legit, did I miss something? it's not the only game either. other games my friend and I have nailed him with an ult and two full Kits (lux Ult, E, Q, AA Combo) and Kaisa Full proc Passive Q isolation and it barely dented him. He wasn't attacking either, so it wasn't any life steal involved, nor was he fed or anything. he just would not take any damage whatsoever. Did I honestly miss something where he has just become this god tank? Cause I'd really like to know so I can at least try and damage him : /