Since the release of 2.6.2, a number of users on the google play store and mobile support google group have been reporting that they need to login everytime they close the app.
So far we have been unable to reproduce the issue locally which makes it hard to debug, but the number of reports for this issue is noticeably high enough to require some deeper digging.
This issue could be in it's own hotfix.
2. Close app
3. Wait for expiration of access_token
4. Open app, should be not logged in