Newbie Question: Why light can shed through corners of my room model?

I’m new to UE4 and now I’m using 4.22 and I have a small house model exported from SketchUp as fbx file (every-plane-twice enabled) and I imported it as static scene with simple color material in a starter kit project. The project just contains a Directional Sunlight and BP_Sky_Sphere. What I don’t understand is that after building there is always some bright area in the room (see pic). I want to know what I’ve missed in order to prevent such artifact. Is this because of my modeling, or the config of lighting / ambient light? or maybe material? Can someone point out links that I can learn more of related topic?