So I tried the 4hr Nurburgring Endurance over the last two days...
I was driving Kaz's N24 Schulze Motor Sports '11 (stock except for oil change & custom transmission and started on RS tires then continued with RH tires).
At about 2hr 10 min in last night, I saved (just as I had done successfully 3-4 times previously) while leading by more than 4 min.
This morning, I loaded my save and the first thing I see is that I'm in position 2 of 12.

I completed my pitstop and rejoined the track. As I went over sector timing lines on my out lap, I received no update on delta behind the 'leading' car. After I completed my out lap, the time deltas started updating again and I was 2 mins behind.
I believe this 'leading' car was a back marker that I would have soon passed had I not saved. After I finish the race, I'll post the make/model of this 'leading' car.
Anyways, I put the throttle down, passed the 'leading' car, and went as fast as possible to build a lead. I'm now up by 3 mins and just saved again at 3hr 10min into the race for what will be my final save. Hopefully, when I reload the save to finish the race, I'll be in first place. Wish me luck!
Anyone else experience this problem?
EDIT:
Race complete and VICTORY! I did not have any new issues when I loaded the save to complete the race. The 'leading' car that skipped into the lead was a Gran Turismo Skyline GT-R '01 which started in 11th. I finished the last hour of the race on SS tires and it was still a great ride.