User unable to logout out due to cache

Problem/Motivation

We are experiencing an issue where the user/logout route appears to be getting cached. It seems to be an issue only on production server (local has cache turned off).

This has two different outcomes depending on if you have 'Log out from Office 365' ticked. In our build we have this ticked and for users who login via SSO this is working as expected. However users login via a Drupal user/password account (i.e not SSO) are not able to logout. They get redirected to the Microsoft screen 'You signed out of your account' but on return to the site they are still logged in.

Deleting the session cookie or clearing the cache seems to resolve the issue until the next user logout.

Steps to reproduce

1. Install on server with cache enabled
2. Login to the site using MS SSO
3. Click logout (user/logout)
4. Login to the site with a standard Drupal user account (i.e not MS SSO)
5. Click logout (user/logout)
6. User redirected to Microsoft sign out screen 'You signed out of your account'
7. Return to site (user still logged in).
8. Turn cache off
9. Drupal account user can now login/logout

Proposed resolution

Fix issue with route cache / cache tags (I guess).