Hello, I am following up on this issue after receiving a response from Apple Developer Support on October 28, 2026. Developer Support advised me to use Apple Developer Forums because they do not provide direct technical support for this issue. However, I have already submitted Feedback Assistant report FB24784951 and contacted Developer Support under cases 102958526964 and 102960544416. The issue remains reproducible: the registered custom-domain Return URL is rejected with “Invalid web redirect url,” while the Firebase-hosted handler URL proceeds to the Apple Account sign-in screen. Could an Apple engineer or a developer experienced with Sign in with Apple review this case and advise what additional diagnostics or configuration checks would help identify the cause? Thank you for your time and assistance.