I've had a look now and my reply is slightly complicated because there is a bug in the code that might sometimes come into action, but even without the bug, the problem is likely to come up.
By the way, I couldn't see anything different for custom configs and I think this could happen in any race.
What is supposed to happen (and does usually happen):
If a result can't be confirmed, because another car that might beat that result is still on track, then if that other car is stationary for 8 seconds, the game decides they will never finish and the result would never be confirmed, so it spectates that player.
So even without the bug I'll also describe, this can cause the problem.
E.g.
A wins the race with 4 laps
B has completed 2 laps but is not moving
C finishes with 2 laps
Problem:
Cannot confirm C's result as B could still beat C. Host spectates B if stationary for 8 seconds.
Separately, the bug (not sure how often this comes up):
1) Suppose the situation is as above except that B only has completed 1 lap. It cannot beat C and is therefore not blocking C's result. It is possible sometimes for the game to spectate B anyway.
2) Suppose the situation is as above except that B has completed 3 laps. Actually in the current system it does block C's result from being confirmed and 'should' be spectated but will not be.
What to do:
For a start, I'll fix the bug, but we are still left with the game spectating cars that are not moving in order to confirm results. What do we need to fix this problem? How about a host option to change that 8 seconds to something else, maybe up to 4 minutes? The easiest way to do this is by a text command after the host is started, and not available on the website. Maybe the default time should be a bit longer than 8 seconds too?