While working on my base, I have noticed that it does not appear possible to place walls on the north side of north-south facing graves. I initially noticed this while trying to place a wall between two graves to close the gaps. I have been using graves to mark off the perimiter where I want to build up a double/hollow wall to surround my safehouse, and the quickest way to wall off the area between the inner walls of the towers and the inner wall of the curtain wall would have been to place the wall between the two graves that happen to line up there. This did not work, so I figured I'd just do it "properly" and build around the walls, getting some work done on the towers in the process. However as I did this I noticed that I still couldn't place walls on the north sides of north-south oriented graves. It doesn't seem to matter if the graves are open or filled, and it only seems to happen on the north side. I don't think I've checked east-west oriented graves, but I don't recall having any trouble with those. I have attached a screenshot of the issue. I made sure I had all the materials and tools required to place a wall frame before taking the picture, so it's unlikely to be a case of PEBCAK (though it might be a case of "a feature, not a bug.") I hope this is enough information?