Very often i'm having this problem and it happened again on the Remus Competition. The day before yesterdsay i set a laptime which was saved as my ghost with the laptime to challenge, let say 1:44:550. Yesterday i managed to drive a 1:44:352 but i when i left the server and rejoined it my ghost time to challenge wasn't the 1:44:352 but it was the older one 1:44:550. Does aynbody knowswhy? I have the replay save on in game.
If you did it immediately, your lap time might have not been updated on the server yet. If you rejoined like half an hour later or more, that sounds like a bug.
I had a lap time not update the other day, also. The better time went on the leaderboard, but my challenge lap was still the previous time and ghost. Not the first time this has happened.