UserDefaults not persisting data

Hi,
We are experiencing what seems to be a fairly widespread, but intermittent issue with one of our apps. Anecdotally I have only seen this in person on iOS 14.4 devices, but I suspect based on our analytics that it is also happening on other versions too.

Basically the UserDefaults seems to be reverting to its initial defaults setting for some users which is causing our pre installed flag to revert to nil. This flag is purely used as a "firstLaunch" detection to determine if the app should clear its cache and show the first time login flow. Therefore this property not saving is having the effect of logging a user out every time they close the app.

I've attached the AppDelegate didFinishLaunching code below, only change is I have redacted our company name and replaced it with MyApp The important part which is intermittently failing is the if statement
if !UserDefaults.standard.bool(forKey: "pre installed") || UserDefaults.standard.bool(forKey: "MyAppSettingsResetState")

For clarity, once an app gets into this state it no longer seems to be able to save to the UserDefaults, so it will always log itself out on next launch. It also seems to fix itself if I delete the app from the phone and reinstall, then the UserDefaults return to working as expected.

Code Block    
func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
     
    SentrySDK.start { options in
      options.dsn = Constants.sentryID
      options.environment = Constants.sentryEnvName
      options.debug = Constants.env == Env.Production ? false : true
    }
     
    _ = MyAppSDK.sharedSDK
    MyAppSDK.sharedSDK.configure(baseURL: Constants.baseURL, gatewayBaseURL: Constants.gatewayBaseURL)
    MyAppSDK.sharedSDK.errorLogging = SentryHelper()
     
    AWSMobileClient.default().initialize { (userState, error) in
      if let err = error {
        print("error: \(err)")
      }
      if let userState = userState {
        print("UserState: \(userState.rawValue)")
      }
    }
     
    if !UserDefaults.standard.bool(forKey: "pre installed") || UserDefaults.standard.bool(forKey: "MyAppSettingsResetState") {
      MyAppSDK.sharedSDK.logout()
      MyAppSDK.sharedSDK.updateAPNToken(token: "")
      UserDefaults.standard.set(true, forKey: "pre installed")
      UserDefaults.standard.set(false, forKey: "MyAppSDKSettingsResetState")
    }
     
    UNUserNotificationCenter.current().getNotificationSettings { (settings) in
      if settings.authorizationStatus == .authorized {
        DispatchQueue.main.async {
          UIApplication.shared.registerForRemoteNotifications()
        }
      }
    }
     
    UNUserNotificationCenter.current().delegate = self
    settingsInfo()
    return true
  }

If anyone has experienced this before and knows how to fix it or what might be causing some users apps to get into this state. I would love to hear from you, I've been going round in circles with this for several days now.