This is pretty strange and haven't heard of the problem before. My question about time is because sometimes, different times (Ex. 12pm) and weather settings in game (Ex. Clear) can begin to make the lights dimmer than other times/weather conditions.
Try locking your time to Afternoon 18 and freeze weather to clear, play the game for a while and see if eventually, the lights get dim. If not, that's your answer.
Also try turning your PostFX in your in game settings to "High" should give you some more brightness to your lights.