I do not believe that abilities that taunt in ESO work the same way as many other games. It seems to just be a large chunk of aggro. Either that or something resets taunted status.
I've noticed it since...well, a month ago. As you say, single target is fine for the most part. It still doesn't keep aggro as long as it says the taunt lasts, which is what leads me to believe that it just slaps on a big chunk of aggro.
Also, you can run around taunting many trash mobs and they just ignore you after the first second. Makes me wonder if you can only have one taunted target at a time, or if it fades when you attack another target.