Syncing key values between devices

synchronize doesn't actually sync data with iCloud, it only syncs in-memory changes with the data that's stores on disk. Synchronizing data with iCloud might take a few seconds, depending on the conditions of your device. It is possible that your second device sees firstRun = false, because it didn't have enough time to perform the initial sync. Try to subscribe to this notification and make sure that it is getting called: https://developer.apple.com/documentation/foundation/nsubiquitouskeyvaluestore/1412267-didchangeexternallynotification

OK, so I can see that the notification is being fired and the value is being changed but the notification appears to be fired after the initial import is done.

From the notification

Code Block 
NSLog("First Run: \(String(describing: notification.userInfo))")

First Run: Optional([AnyHashable("NSUbiquitousKeyValueStoreChangedKeysKey"): <__NSArrayM 0x60000391c450>( firstRun, timeGrouping, imageThumbSize, dbVersion ) , AnyHashable("NSUbiquitousKeyValueStoreChangeReasonKey"): 1])

Launching the application

Code Block 
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
		appData.moc = persistentContainer.viewContext
		NotificationCenter.default.addObserver(self, selector: #selector(onUbiquitousKeyValueStoreDidChangeExternally(notification:)), name: NSUbiquitousKeyValueStore.didChangeExternallyNotification, object: NSUbiquitousKeyValueStore.default)
		performUpdates()
		return true
 }

and the performUpdates function

Code Block 
		func performUpdates() {
        let firstRunPref: Bool = prefs.bool(forKey: "firstRun")
        let firstRun: Bool = keyValStore.bool(forKey: "firstRun")
        if(!firstRun) {
            // Check for old prefs, if existing, there is already data in the db
            if(!firstRunPref) {
                // No previous data
                keyValStore.set(true, forKey: "firstRun")
                NSLog("First Run: Yes inital keyval, no prefs, initial import")
            } else {
                // Previous data exists
                keyValStore.set(true, forKey: "firstRun")
                NSLog("First Run: Yes inital keyval, previous prefs, convert")
            }
        } else {
            updates(useKeyVal: true)
            NSLog("First Run: No, update")
        }
    }