I also want to add there is an error in the Xcode console on startup of app: Could not create a sandbox extension for '/var/containers/Bundle/Application/..... Both build settings Use Script Sandboxing and Enable App Sandbox are set to NO. Could be that whatever is causing this error is preventing writing the cookies?

This is a late response but thought I would reply in case anyone else has the question. According to developer documentation it doesn't seem possible for macOS app on App Store. See link to documentation. To distribute a macOS app through the Mac App Store, you must enable the App Sandbox capability. https://developer.apple.com/documentation/security/app_sandbox?language=objc

After further debugging this seems related to iOS 18 and Safari not able to read the cookies. We still have no resolution.

Issue documented in FB14862013