Don't get Firebase A/B draft experiment remote config value on my test devices

I'd like to create a Firebase A/B test experiment and tweak the setup before starting it by using the option to add a test device via its Firebase Instance ID, however, I never seem to get the Remote Config param for the experiment.

The fetch is triggered in applicationDidFinishLaunching of the AppDelegate:

        let remoteConfig = RemoteConfig.remoteConfig()
    #if DEBUG
        let expirationDuration: TimeInterval = 0
        remoteConfig.configSettings = RemoteConfigSettings(developerModeEnabled: true)
    #else
        let expirationDuration: TimeInterval = 3600
    #endif

    remoteConfig.fetch(withExpirationDuration: expirationDuration) { status, error in
        if let error = error {
            RLogError("FirebaseHelper >>> Error fetching config: \(error)")
        }

        RLogInfo("FirebaseHelper >>> Config fetch completed with status: \(status)")
        self.activateRemoteConfig()
    }

After activating the remote config in the callback the param is never there. If I start the experiment, however, the value appears. No matter if the app is re-launched multiple times or I uninstall and re-install the app and re-add the Instance ID to the Draft Experiment.

Any help is appreciated.

Turned out that my issues were related to the Firebase configuration which was failing on iOS Release builds. Rolling back to a fixed 5.15.0 version (current is 5.16.0 at the time of writing) resolved all issues I've had for now.