Could it be the sunlight is reflecting off of radioactive particles in the high atmosphere?
This was my initial thought, but a little bit of research turned up a simple reality of nuclear fallout: it's invisible. It doesn't emit light. So that's not it.
Plus, it's the same in The Witcher Series, though that I excused as being northern enough in latitude that the days might simply be longer...except that this should then happen during summer, not autumn. (Unless the "Northern Realms" are in the southern hemisphere of the planet.)
Thus, I must think this is either an engine goof, or a quirky design decision.
I now excuse it in both games as follows:
Geralt has witcher senses, which increase his eyes' sensitivity to light. Thus, that's how his cat-eyes see the sky at 2:30 am. Whereas V has augmented eyes. It's like he's a bloody witcher or something.
And then my brain can relax and the suspension of disbelief can settle like a warm blankey as I
proceed to read a newspaper from across the room at 2:30 am with the lights off.