It might be better to suggest that the real soul has moved on to the afterlife, and the reanimation of the body has left it open to possession by an evil spirit of some kind, one that gives it enough volition to walk around and munch brains. Maybe even a spirit that knows enough about the host-body to mimic some of its habits in life.
I'm cool with that explanation.
IMHO, the soul is never forcibly attached to the body. The host dies, the soul goes to the afterlife. So the zombie cannot therefore be conscious. Either it's a mindless husk, not controlled by anything at all and without consciousness, or it's an alien/demon possessing it.