It looks like AI drivers stop for fuel on races measured in laps, but not on timed races. I ran a few races today that were 100 or more laps and each time the AI would pit for fuel (or at least try to). I just ran a 1 hour race on Westhill with three each of the GTR class cars with "pit stop required"=yes. The AI cars correctly determined that they would need to run 2 stints, and they put less than a full tank of fuel in. However, by lap 22 (about 35 minutes into the hour long race) the FXRs were running out of fuel. Two laps later the XRRs and FZRs ran out. None of the cars ever stopped for fuel.
So, they correctly knew that they would have to stop for fuel. They correctly gave themselves enough fuel for about half of the distance. But they never stopped to refill.