walkers dont use a camera to see like the player model does.
Walkers probably use a trigger zone, if you are in that trigger zone, they know you are there and will target you. The walls make it more complex by creating fields of LOS that tells the walker if they can or cannot hit you. So if can see a toe (or rather if you can see the walker in any way) you are in LOS even if you are standing in a way that the wall still blocks the damage from the attack.
Could be wrong since i dont have access to actual dev files but that is the most likely thing that is happening.