2014 C250 Brake light warning on not working

Some bulbs are dual filament so one part of the bulb may be lighting up and the other is not. They physically need to be removed and tested with a basic continuity test. Would make sure the offending side or bulb be replaced with some dielectric grease and positioned properly in the hole. If all the above checks out there may be an issue elsewhere and may have to be scanned with a factory STAR tool. I don't own an M-class but had an older Benz for 15 years. Yes they can be quirky.

I have a 2014 C300 that was doing the same thing. You need a heavier ground wire attached to the right brake light ground plate. I did this 1 year ago and no more problems. Mercedes charges $200.00 or so to do this, but if you are mechanically inclined you do it in less than 30 minutes. there are videos on youtube, from a mercedes engineer in Germany that shows how.