Taunts not working on Stivastin MM (Manaan last boss)

Last night, I was running Depths of Manaan in MM as a tank, and made it to the last boss. Right after the aggro dump that knocks the tank back, as a powertech tank, I tried taunting Stivastin back right after the fact, and couldn't get him back, even though there is no mechanic that targets either at random (like the last boss of Kaon or HK-47 in Foundry) or renders the boss untauntable for a certain period of time.

Did anyone else run into the issue of taunts that don't work on Stivastin in MM on a different spec? On a different difficulty level (namely VM)?

There actually is a mechanic in place that you can't taunt him back immediately, you need to wait for the debuff that you get at the same time with the knock-back to fall off before your taunts work properly again. I think the info text says something like your threat is ignored or so. So as frequent as that boss is kicking you away and puts that debuff on you, it's pretty normal that the person second highest in threat 'tanks' for almost half the fight.