When I test the app using a TestFlight version built by the CI machine, the device receives push notifications.
But when I rebuild the app under Xcode locally with the debugger connected, the device doesn't receive any push notifications.
To validate things further I again tested locally with a push notifications tester, and the device indeed received a push notification.
So the code isn't omitted due to it being a #DEBUG build as the grant and registrations pass.
What is my local build possibly missing?
When you build your app in debug locally your app will generate a unique device token that doesn't receive production push notifications. When you build your app with a distribution provisioning profile it generates a different device token that does work on production.
Apple will reject a development device token unless you are specifically requesting a push notification using your APNS Sandbox certificate.
So if you haven't already you'll have to go through the certificate creation process all over again but this time for a sandbox certificate and use that one instead.
The solution ended up being that our server with the Houston gem needed to configure its APNS support to be in development mode, and not in production mode. 👍