The planet's surface basically has one of x different overlays on to create the darker effect. The downside as you've noticed is it does apply to the entire surface, including actual lights!
It might be possible to lessen the impact on the lamps by making a more precise LUT but it's not my area of expertise and I'm not optimistic.
I did previously explore an option where I manipulated the time of day to skip over varying amounts of the day, this lets lamps work properly but has other side effects...
Oh and I believe I already have settings to tune when lamps turn on/off, but I don't think it can be done per planet.
I'll look into it again, thanks for the report!