In a new restart of the game, I exercised a little creative license in the Summer portion of the Prologue. Rather than use the starter car offered to me in Round 1 of the Dirt Racing series, I instead Quit the race, changed to the road racing starter car, and then continued through the Prologue using that car instead.
Now that I played through the Prologue and qualified for the roster (entering Horizon Life proper and synching up with the world), every time I try to open a screen in my Creative Hub (accessing old tunes or blueprints) I get the message “This feature is locked. Qualify for Autumn and complete the first event at Tarn Hows…”
I fear that by deviating from the script of the Prologue, I kept the game from flagging a particular event as complete.