I have the same issue.
Just had my P3 service so my brake fluid level is at max, the brake lights (sometimes) work when I catch them in the reflection of windows/other cars, etc. However, if I lightly apply the brakes, the 3 lights on the dashboard don't appear, when I brake normally, or hard, they do.
Going to check the bulbs in the lights & the fuses, if not, I assume it'll be the brake light switch, maybe worse?