Can anybody tell me what am I missing? Using GPU Lightmass UE5.3
Most likely a mesh issue not a material issue. Unless you require two sided materials for some reason and your material is set to one sided.
I’m not saying that every material should be two sided. By default unreal materials are one sided while 3ds max materials are always two sided. This can be confusing.
Maybe your wall is just a plane and not a box or it has inverted polygons and the light shines through from the other side.
Hope that makes sense.