Any lucky? I’m not convinced that it is a fly.io issue. I’m having similar problems when I dockerize the app on AWS elasticbeanstalk. Something with the way the shopify-api handles things during development is not syncing with a build release.
@Shaf1 Make sure your fly.io url is actually https. When I first spun up my server it did not give me an SSL cert, I had to set up my domain to get my cert; as you may know, Shopify requires a SSL.