I raced my brother online doing a couple of F1 seasons a few years ago. I do remember this problem being mentioned, I don't remember us having it however. I think that the solution might be to set the race type to distance, rather than laps or time. We used to race 60 miles, around 15-20 laps at most tracks, and didn't have that problem, and it also means that the AI put in enough fuel to get to the end. Choosing time, say a 30 minute race, they'd always pit on the last lap, the fuel calculations were bugged.
It's a long shot, but I hope that you manage to solve it.