Since upgrading to iOS 11, our web app is persisting expired cookies. It only happens when adding the app to home screen. We built a simple web app to just login/logout. Stackoverflow post with code example can be found here:
https://stackoverflow.com/questions/46551996/ios-11-cookie-expiration-in-home-screen-web-apps
It adds a persistent cookie at login. At logout, it expires the cookie. If you logout, close the app by hitting the home screen button, then launch the app again from the home screen it will occasionally log you right back in because the cookie is still present and being passed in the request.
At times it will also do the opposite where it deletes the persistent cookie, thus logging the user out. To reproduce we just launch the app from home screen, login, tap home screen button to close the app, then relaunch the application. It launches and asks the user to log back in because the cookie was not passed in the request to login.