Debugging a 400 Error in Sign in with Apple Token Exchange

Today, I submitted a new build of my app to App Store Connect. While working on the Apple Sign In integration for this Flutter project, I ran into a server-side issue during the token exchange process. For a long time, I left this project without a login feature. In the beginning of this project, I wanted users to access all features freely, without strict authentication. In the Google Play version, users can sign in with Google, but the App Store version didn’t support Apple Sign In. At the time, this was just a personal project. After releasing it, I got a job and didn’t really have time to maintain or improve it. So the Google login stayed, but Apple Sign In was never properly implemented. Anyway, I finally decided to implement it. I went back to my legacy code. Some parts were already there, except the withdrawal flow. I implemented Apple Sign In, logout, and auto-login. (If Apple Sign In is successful, my Node.js server responds with an access token to the Flutter front end, and t