Another light bleeding question

Is there a way to make walls which are thinner than 40 units not bleed? As a test, I made a cube within a cube both of which are water tight. The scene has a single dynamic directional light. What I found is if you make the wall thickness less than 40 units, you get light bleeding. A wall that’s 10 units bleeds a lot. This is really creating problems for some buildings I’m trying to make which have corrugated metal skins which are rather thin. I played with the shadow bias which helped some. But that produced a lot of artifacts.

I’ve had exactly the same thing.

I seem to recall moveable directional and skylight helped a lot. Also tweaking various shadow params on the directional.