Ok, so for some bizarre reason, when I extend the roof, the shadow problem disappears. It’s really weird, and I don’t have a proper explanation for that. I’ll provide that as the solution…
Since Roblox global lighting seems to be more voxel based (4x4x4 studs) light tends to bleed through thinner walls at the corners. If you change the wall thickness to 4 studs it’s not as much of an issue.