It happened to me once last year (before Dreamsurge stuff) and it only happened on one character. I reported it to Blizzard as a bug, and since then I’ve not seen any races in that zone.
Update: I just one of ther races in Zaralek Cavern and even thou the WQ says I should get a purse I received no reward.
Race was “Dragonrider Racing - Brimstone Scramble”
Note: I also turned off all add-ons and the race still shows as if it wasn’t completed.
But Report: I just posted a bug report for Blizzard, in case anyone has the same issue you can chime in Dragon racing no reward - bug