Jockeys get to pick their mounts, so if two people requested your jockey of choice, he may have decided to go with the other horse. Then your horse would try to get it’s 2nd choice unless that jockey picked a different horse, etc etc.
When this has happened, my 1st choice jock did not ride another horse in that race. It is not that he chose another horse over mine. I understand that but it is frustrating if your horse has done good under that rider and you request him and he doesn’t ride at all in that race.
It has been sporadic and it seems to be on J. Knoll. It is not a high priority to me but I just tossed it out there in case it was some hidden code thing.