i would say its your brake switch, the LED draws less current than the lower incandescent bulbs so that may explain it..not sure

Careful when removing/adjusting that switch, there is a video on yt. If my memory still works, the lower brake lights are controlled by the REM (google up) while the upper one is by the CEM Possibly there is a faulty relay in the REM, it appears to be RM15 or RSH6 https://www.matthewsvolvosite.com/f...