So I played a few races with the 'mandatory' pit stops and wondered why I pitted early in a good position, but finished the race near last. And then figured out why : most players never pitted at all.
So in the next few races with pit stops, I did not pit. Some did not disqualify me at the end of the race, some did. (Seemed to be the races with higher number of the laps that dq'd me. The couple with only 5 laps did not dq me.