The game is absolute acting like I've already done it, but not showing that I've done it in the Accolades screen.
Pinning the challenge brings up three checkboxes:
Get 3 Stars at Trailblazer A
Get 3 Stars at Trailblazer B
Complete in under 3'30
They appear on screen for a fraction of a second before the third box is ticked, the Accolade appears above them and is ticked, and it all vanishes. The Accolade menu shows it uncompleted.
Completing Trailblazer A brings all three boxes back up on screen again, with Trailblazer A/3'30 ticked, and the Accolade appears above them again and is ticked, and it all vanishes. The Accolade menu shows it uncompleted.
Finally, completing Trailblazer B only brings the Trailblazer B box up, and it's ticked. Then the Accolade appears above and is ticked. Then, at long last, the Accolade menu shows it completed.
Weirdly, the time requirement is gone entirely because it's already complete. You can just amble round and complete the two Trailblazers in the three-star time.
I think it's fair to say there's a few kinks in the Accolade system to iron out!
Edit: Going back over older ones, it looks like ALL my combo Accolades achieved have been reset to uncompleted with the game still thinking I've done them but not displaying it as done in the Accolades menu.
I can also confirm, having redone my Danger Signs ones (oh, also, all the "50m" Danger Sign Accolades are broken) that recompleting them gives new Accolade points. I hadn't noticed that before.