I’ve had this. What you’ll probably find is the race you have yet to complete is part of the weekly Festival playlist and isn’t showing up on the map as a normal race event. You need to change the championship to a regular event when you start the championship. So for this week go to Dancing in the Dirt championship (Meridian), enter the event then press the Y key to change to a normal race. Do the race and the accolade should unlock.
Hope this helps.