ToB - Backstab

I completed SoA with a Thief as my main character, but as soon as Throne of Bhaal started, I could no longer successfully backstab; enemies would turn in my direction, regardless of whether I was using Stealth, or (Improved) Invisibility, or an invisibility potion.

Is this a bug, or intended behavior? Any workarounds or advice for thieves wanting to slide a dagger into the ribs of another big bad Bhaalspawn?