Ideas
I have an additional bit of in-game stuff that might lead to a diagnosis.
I think maybe my "the game is confused, literally, about what coutns" theory (I mean, no duh, but. still) might be bolstered by the fact that arches appear to be broken at the moment.
Make two rooms with no windows. Place an arch between them...and they'll light up a bit. Place an arch from a well lit room into a dimly lit room, and the light won't be correct whatsoever. Or place an arch from the outside into a room, and the light inside will be bright as heck, EVEN IF the arch is on the side of the building in the shade, or it's cloudy outside!
Is this a related issue? Arches have always "produced light" because it's just how the "make light go indoors" system worked before the 2016 lighting update, but they did so logically. Lately they seem to just generate light in all situations.