Think damag calculations involving mr are off

Played this game where I took 3 seconds to die unfocused and effectively had the highest mr I could at that point in the game. I don't think they had staking penetration yet so I'm really confused as to why I should die that quickly. Also garen was doing 1800 damage using r when I was at 1/4 health. I did not have that high of missing health. How can I help track down this issue? Are there files I can submit? I lost the game because I could not tank being even being focused with two mages as they ripped me apart like I wasn't adequately resisting.