This has happened twice to me now, let me try to explain what happens and why I think it's happening. After completing the first round of the first race of the C class Fujimi Kaido trial the 2nd place car would disappear as soon as I crossed the line, upon loading round 2 I was racing against the same car. Once I finished this race and crossed the line my Xbox would completely freeze with the progression of the race saying 99% complete. After restarting my Xbox and trying again I realised that the second round spawned me a different model car to race against and the 2nd place AI from round 1 didn't disappear at the end of the round. 

So I'm guessing this is a respawning issue with the round 1 car spawning too early or an error in changing the cars around ready for round 2, either way Forza has done this twice to me now and it's making it frustrating when trying to finish this C class series.