I find the infested cellar to be unusual in that it is two stories deep. Maybe the owner tried to wall off the infested portion earlier and when that failed decided to call the exterminators... Or maybe a previous owner used the walled off section to store contraband.
I don't think it is a bug; I think it is an optional/random feature of the Infested Cellar mission.
..or amybe tjhose blocked, closed part of cellar was oldest, and .."deepest", and as such was haunted and "Zone" infested - the previous´ly owners often experienced there haunting ghost, mutanted (radiated) eeries, and creepy creatures, and various mysteriously, fringe ..events and oddy, unexplicated *Zone" - phenomenas ..
..so they closed it all, excluded the horrible area with ferro-concreted, /bricks´ed strong wall..
i definately would like such mission.. (ghaunted cellars, ghosted dungeon..)