So after you get out of the tunnels in the darkness, after the first grim. You go up the stairs next to the dock - the long hallway leading up to the plank on which you jump over the cart blocking the way.
If you firewalk here in this stairwell, you will end up stuck in the corner above the stairwell (top rightish). Only way out is to overcharge out, since you lose control of movement and sienna wallows down.
Sorry for not having a SS, but it has happend two times now.