The probem is that when the back button is used, the browser uses its local history to re show the page without checking with the server if it is correct.
I recently provided two solutions here https://stackoverflow.com/questions/59903552/laravel-logout-hittin-gthe-back-button-send-s-me-back-into-the-app-how-do-i-pr
one disables browser caching of content, the other disables the back button