Hello,
This could me me missing something, but I can't get the endpoint api.itch.io/uploads/12748042/download (for this example, that's celeste-linux.zip) to recognise that I own the game.
https://partlyawesome.itch.io/

https://api.itch.io/profile/owned-keys

However:
hayley@main /e/nixos (master)> curl -X GET "https://api.itch.io/uploads/12748042/download?api_key=06[so-on-so-forth]" -H
"Authorization: Bearer 06[so-on-so-forth]"
{"errors":["you must buy this game to download"]}
Am I missing something? Let me know if any of my above assumptions are incorrect.