I'm not 100% certain, but this might be normal behavior across all versions of the game.We have this fixed and will be a part of the next update.It's not as easy as darkening a part of the texture file it has to do with how the game considers that piece of metal as part of the top-most floor (where the fog fade has influence).
So much so, that it's safer that we don't touch this. The way this room works/loads is different and more complicated than other rooms. So much so that I asked Bigmanjapan about it.
Hi for all these notes! I should be able to answer most/all of them: