@Ceciliabr

Is there any rational explanation for this peculiar abnormality?

This is a "normal" behavior of the renderer.

The default global illumination uses Ambient Occlusion and its parameters allow to have at daytime a minimum luminosity in a room completely closed.
So at night, without additional lighting, the image is black.

I changed the behavior of the default global illumination to have a minimum nighttime brightness for a sky texture.
This option is disabled for a sky color because it changes to a blue hue that varies with time.

Dark value = 1/255
Daytime
Nighttime

Dark value = 2/255
Daytime
Nighttime

What result (value) do you prefer?


Here is another example without light, at night.
Dark value = 1/255
Dark value = 2/255