Hello, Saurav,
This is kind of shot in the dark, but have could you try closing the InAppBrowser from the app's side, instead of doing it from the callback page? You can use deep links to trigger a screen redirection, which in turn will run the logic to close the window.
If it does not solve the situation, have you tried debugging your app using Chrome? Through this method, you might be able to understand if the InAppBrowser window was actually closed.
Please let us know if you are able to overcome your situation.
Best regards,