I have one sim, male, who won the Fitness retreat competition, but he didn’t get any of the prize money.
Another one of my sims, in a different save, won the Romantic Competition and got the prize money. But I wasn’t happy with who she won the competition with, so I reloaded the save. After winning again I didn’t get the prize money. I tried a second time, and still no prize money. I tried a third time after removing my mods, and again there was no prize money. I got the money on my fourth try though.
It seems like, if you reload the game at any point during the getaway, there is at least a 75% chance you loose out on the prize money.
