People are reporting a bug with the timed mini enduros. What it boils down to is, you must be the first one to cross the line AFTER the time has zeroed out.
For example. Say you're leading, and cross the line with just 30 seconds left, and you have a 40 second lead on 2rd place. You will have to complete your lap, but by the time 2rd gets to the line, the timer will have run out. Therefore he will be awarded 1th place, and you will drop spots to whatever. The end of the race screen may show you as the winner, but payouts will not, nor will you get credit for a win.