If you did it today, and not before 2.0, the assassin didn't break cc in this fight. These bosses can't be cced anymore. Your cc just failed because the bosses were immune.
Guess you are right, I was healing so I didn't look to see if they were immune, besides the 2 seconds it took me to cast whirlwind, which I ignored any buffs I saw, assuming we could still cc ( I have not done sm in a while). I just saw the merc mark a cc and the assassin attack that target right away, almost right after the merc casted his skill. I guess we just assumed he broke all our ccs