I'm experiencing a strange issue where ASWebAuthenticationSession works perfectly when running from Xcode (both Debug and Release), but fails on TestFlight builds. The setup: iOS app using ASWebAuthenticationSession for OIDC login (Keycloak) Custom URL scheme callback (myapp://) prefersEphemeralWebBrowserSession = false The issue: When using iOS Keychain autofill (with Face ID/Touch ID or normal iphone pw, that auto-submits the form) -> works perfectly When manually typing credentials and clicking the login button -> fails with white screen When it fails, the form POST from Keycloak back to my server (/signin-oidc) never reaches the server at all. The authentication session just shows a white screen. Reproduced on: Multiple devices (iPhone 15 Pro, etc.) iOS 18.x Xcode 16.x Multiple TestFlight testers confirmed same behavior What I've tried: Clearing Safari cookies/data prefersEphemeralWebBrowserSession = true and false Different SameSite cookie policies on server Verified custom URL scheme is registered and works (testing myapp://test in Safari opens the app) Why custom URL scheme instead of Universal Links: We couldn't get Universal Links to trigger from a js redirect (window.location.href) within ASWebAuthenticationSession. Only custom URL schemes seemed to be intercepted. If there's a way to make Universal Links work in this context, without a manual user-interaction we'd be happy to try. iOS Keychain autofill works The only working path is iOS Keychain autofill that requires iphone-authentication and auto-submits the form. Any manual form submission fails, but only on TestFlight - not Xcode builds. Has anyone encountered this or know a workaround?