Kudos to MattW_CFI for discovering this bug
Here's a further comment from eie from elsewhere in the forum:
eie wrote:Tip: You can have as many tries as you want on any map, and still be able to unlock the next won. (No need to hunt for medals )
Edit: Apparantly you have to achieve certain medals to unlock the tracks, and you apparantly need all gold medals to unlock the final map, although you can just click "go to next map" whenever you finish a map to avoid this :p