That's odd. What device are you on? Also, have you tried using a different device to change the thumbnail? Let us know and we'll take a further look.
You can add location information to your Tweets, such as your city or precise location, from the web and via third-party applications. You always have the option to delete your Tweet location history. Learn more
That's odd. What device are you on? Also, have you tried using a different device to change the thumbnail? Let us know and we'll take a further look.