different, visual appearances of same binary app installed on same iOS version

Recently, I have released a new version iPhone app, but as the following pictures, the back button of pages and the general button respectively have difference appearances in spite of the same iOS version(v8.4)

I must admit that I do not have deep understanding about iPhone app, and my iPhone app has been built on the Xamarin platform.

Does anyone know why this is occurred?

Back button

general button