For many cars the brake lights are nearly invisible. The difference in brightness of lights / brake lights is often close to zero (night/rain). Additionally the effect of sunlight to the lights / brake lights is very high and often unrealistic (lights are in the shadow, but glow in sun reflection).
The judgement if a car is braking is often nearly impossible.
While I do agree the brake lights and even headlights look a little brighter than you would expect IRL during the day, I’ve had no problems at all telling when the AI cars are braking and when they’re not, be it night, day or rain.
What picture settings are you using? Do you have enhancements on?
No special adjustemts, everything else looks very good
If you’re looking for brake lights in public lobbies-you won’t find them! 
I’ve noticed this when following some cars, but haven’t kept track of which cars have the harder to see brake lights.