Hello,
I file this under bugs as I don't think it is as it should be.
Even in the new engine, outside light will illuminate rooms and corridors inside buildings that have no windows. This is not only unrealistic but it does also impact the gameplay. If indoor illumination would be realistic the flashlight would be a much more important tool (even in daytime) and the game would have far more athmosphere. Players/Zombies could hide in dark corners e.t.a.