Yasuo is taking no damage from morde's ult

I'm not sure if it's a bug or if it's meant to be like that, but since morde is reputed to have a lot of bugs, I thought I could ask here.

I just did a game, with morde and the other team had a yasuo. I wasn't laning against him, so I just noticed that like 2 or 3 times during the whole game, and I can't reproduce it in a bot match since yasuo can't be played as a bot.

It says that morde's ult deals 35% (when max) plus 4% of ability powers. I had a little more than 270 AP (total of AP from items), so it's a minimum of 10.8% in addition of the 35% which makes 45.8%. In addition of that, I had liandry, which add 10% more (2% per seconds), so my ult is suppose to deal 55.8% of his max health. He only had 50-52 magic resist (didn't buy any).

However, if Yasuo has his shield and I ult him, he'll take absolutely no damage from it... without attacking anything. Actually, he'll lost his shield when my ult will be over.

I don't know if it has something to do with the fact that I deal damage base on max health and he has a shield so the game goes crazy or if it's another bug or if there's something I don't understand...