Problem with armor and tenacity

Hey there,

1st of all, I want to know more about armor. I just played a game with Malphite in aram facing 3 adcs (Graves, twitch and Kalista) They were all critical. I stacked 461 armor / 3100 Hp with Thornmail and for some unkown reasons, whoever attacking me wasn't losing any hp. Thornmail is bugged or I'm the only player who can die by a thornmail ?

Match history : http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1910352895/202331272?tab=stats

One last thing, Garen's silence last 5sec.. with mercury's boots I'm suppose to be silenced for around 3-3.5 sec but I still got silenced for over 5 seconds. Is my client broken or armor/tenacity has changed ?