Maybe that happened because there was no other way around it.
I just tried to make my group run through a lefotver burning area and poison sludge - (after i cleared those three zombies behind a wagon, just outside northern gate) - and they all went around the surfaces, whichever way i tried to make them go.
The only thing that happened a few times is that one of the characters just barely zinged some poison sludge surface and got some damage that way.
Now, even if they group up into some narrow passage - one companion stops and waits for others to pass - so he can then go - instead of just running over on the side.
There is no setting for it. I think you just might have found one place where they dont perform as well.
