Hello, I am experiencing a persistent Sign in with Apple issue for our production web application, Sai＊aus. When Sign in with Apple is initiated using our registered custom-domain return URL, Apple returns: invalid_request Invalid web redirect url. Configuration: Services ID: com.saiaus.app.auth Production domain: sai-aus.com Registered Return URL: https://sai-aus.com/__/auth/handler We have verified that the redirect_uri sent in the authorization request exactly matches the registered Return URL. As a comparison test, using the Firebase-hosted handler URL https://sai-aus.firebaseapp.com/__/auth/handler with the same Services ID proceeds successfully to the Apple Account sign-in screen. The failure occurs with the registered custom-domain handler. We previously contacted Apple Developer Support regarding this issue: Support Case: 102958526964 Support Case: 102960544416 Feedback Assistant: FB24784951 We submitted the issue through Feedback Assistant as directed and also provided follow-up information, but the issue remains unresolved. We tested the production flow again on October 4, 2026, and the same invalid_request / Invalid web redirect url. error is still reproducible. Could an Apple engineer please review FB24784951 and advise why the registered custom-domain Return URL is being rejected while the Firebase-hosted Return URL works? Thank you.