And, when you say half a lap, do you mean that they will travel for an additional half lap and then stop dead? '
Or, is it that all cars are stopping once the last car to cross after the leader has done so?
I think the default is for the race to end when all cars have crossed after the leader, rather than all cars finishing all the laps. As such, it can seem like some cars get to keep going while others just stop after the line, or whatever.
If you want all cars to complete all laps, then you need to change the Race End option on the Single Event screen, in the Event section at the bottom left, to "All cars finished race(Only GP)". That will ensure that all cars in a lap limited (GP) race complete all the laps of the race, even if they are many laps behind the leader.