I have a theory. I'm currently doing laps with a CronusMax because I'm not going to sit there doing laps at Indy for 95+ hours. So I have my CronusMax doing 3 lap races and restarting the race at the end. Everything is fine most of the time but sometimes I'll wake up in the morning and my Xbox will be reset and back at the dashboard. I'm assuming it has installed an update and restarted.
I think what's happening is that as long as the game is running, it's counting the miles and moving the tracker appropriately, but I'm guessing that when you restart a race at the end rather than continue, it doesn't autosave, so if something happens like the game getting cut off because the Xbox restarts itself, you lose any progress you made since the last autosave. The tracker stays where it is, but the miles traveled in the game revert back to the last time it saved, so then you have a period of potentially many hours where the game is counting up miles but it's still below where the achievement tracker is currently sitting, so you don't see any progress on the tracker until it "catches up" to point where the tracker is and will start moving the tracker again.
We could verify this for sure if we had a display of miles traveled in the game, but we don't so it's all speculation.
I leave the game running and go to bed and the tracker is at 82% (roughly 20,500 miles). During the night it keeps going and moves the tracker to 85% (roughly 21,250 miles). I wake up in the morning and find my Xbox has reset to the dashboard, likely because of an update. I launch the game and start doing laps again and do 500 miles which should move the tracker 2% but the tracker doesn't move. This is because as far as the game is concerned, I only have 21,000 miles driven since I lost the 750 that didn't get save before the Xbox reset, so until I get back to 21,250 miles, the tracker won't move again.
If the CronusMax has been running for 18 hours before this happens, that means you lose 18 hours worth of progress and the tracker won't start moving again until you redo that previous 18 hours. So I guess the lesson is make sure you end races periodically to get it to autosave often so you don't get screwed by a power outage or an update or what have you rather than constantly restarting the same race.