Light bleeding from a hollow BSP brush

I have a simple corridor and a room in my map which i make out of Goemetry>Box brush and setting Hollow to true.

If I have no static lights in my scene i get a beautiful dark room and corridor as expected.
If i have static spot or point lights no problem.

But then I added a directional light outside my enclosed room and map.
I get a weird kind of light bleed from the edges where two walls meet.

See attached picture.

This looks crazy because these two walls are made my the engine itself, by Hollow-ing out a BSP brush.
So this has to be a bug in the engine.