We had AX event (autocross / arena / custom layout / 1 lap), server was dedi T, my client was U.
Each player drove 1 "lap" and then spectate, after he joins spectators, "Unknown finisher" message appears - no real problem, results was correct.
Another thing, after race end we started race for round2 and results was still there, we had to change track to wipe off previous results.

This is not a bug, this is related to the way LFS calculates the final results. It sends some packets with the racername after the race, and if not found on track (ie Spectating, or Quit), it writes "Unknown Finisher" on screen. (The results you see on screen are correct, but for example if you use stats! (by smith), it would say DNF for the racer)

I tried some races with AIs and it says "unknown finisher" if I join spectators imediatelly after finish, but it was with every racer in our AX event, for example 20sec after finish.
Maybe this is related to way how results are calculated, but persisting results from race to race is imo bug.