Authentication failure upgrading Ember from 2.7 to 2.8

I'm upgrading an app from Ember 2.7 to 2.8 and am having some odd test failures. I use ember-simple-auth for authentication, I test this in an acceptance test by filling in my login form appropriately with a variable email then using:

equal(currentSession(application).get('data.authenticated.user_email'), email, "Successfully logged in " + email);

This works fine in 2.7 but in 2.8 the variable currentSession(application).get('data.authenticated.user_email') sometimes returns undefined. This happens intermittently - I have a test which logs in several different users and if I run the test suite multiple times, different users fail to log in each time.

I'm quite confused by this - any ideas would be gratefully appreciated!