EmberJS test fails first time running in phantomjs

Problem

I have a /login route that uses ember-simple-auth to implement authentication. During testing ember-cli-mirage is used to mock the backend. The user logs in by providing their email address and password.

In total I have 4 acceptance tests for this route, similar to the test below:

test('should show error message for invalid email', function(assert) {
  visit('/login');
  fillIn('input#email', 'invalid-email');
  fillIn('input#password', 'invalid-password');
  click('button.button');
  andThen(function() {
    assert.equal(find('div.notification').text(), "Invalid email/password");
  });
});

When I run the tests using ember t only the first test in the file fails. If I comment this test out, the next one fails, and so on. If I run the tests in server mode with ember t -s the same test fails; however, when I press enter to re-run the tests, all the tests pass.

The failure message is always the same, shown below:

not ok 7 PhantomJS 2.1 - Acceptance | login: should show error message for invalid email
    ---
        actual: >

        expected: >
            Invalid email/password
        stack: >
            http://localhost:7357/assets/tests.js:22:19
            andThen@http://localhost:7357/assets/vendor.js:48231:41
            http://localhost:7357/assets/vendor.js:48174:24
            isolate@http://localhost:7357/assets/vendor.js:49302:30
            http://localhost:7357/assets/vendor.js:49258:23
            tryCatch@http://localhost:7357/assets/vendor.js:68726:20
            invokeCallback@http://localhost:7357/assets/vendor.js:68738:21
            publish@http://localhost:7357/assets/vendor.js:68709:21
            http://localhost:7357/assets/vendor.js:48192:24
            invoke@http://localhost:7357/assets/vendor.js:10892:18
            flush@http://localhost:7357/assets/vendor.js:10960:15
            flush@http://localhost:7357/assets/vendor.js:11084:20
            end@http://localhost:7357/assets/vendor.js:11154:28
            run@http://localhost:7357/assets/vendor.js:11277:19
            run@http://localhost:7357/assets/vendor.js:32073:32
            http://localhost:7357/assets/vendor.js:48783:24
        Log: |

After all the tests have run, test emits an exception:

# tests 60
# pass  59
# skip  0
# fail  1
Not all tests passed.
Error: Not all tests passed.
    at EventEmitter.getExitCode (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/lib/app.js:434:15)
    at EventEmitter.exit (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/lib/app.js:189:23)
    at /home/jon/projects/jonblack/wishlist-web/node_modules/testem/lib/app.js:103:14
    at tryCatcher (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/util.js:16:23)
    at Promise._settlePromiseFromHandler (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/promise.js:510:31)
    at Promise._settlePromise (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/promise.js:567:18)
    at Promise._settlePromise0 (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/promise.js:612:10)
    at Promise._settlePromises (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/promise.js:691:18)
    at Async._drainQueue (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/async.js:138:16)
    at Async._drainQueues (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/async.js:148:10)
    at Immediate.Async.drainQueues (/home/jon/projects/jonblack/wishlist-web/node_modules/testem/node_modules/bluebird/js/release/async.js:17:14)
    at runCallback (timers.js:637:20)
    at tryOnImmediate (timers.js:610:5)
    at processImmediate [as _immediateCallback] (timers.js:582:5)

It seems odd that this is emitted for tests failing rather than just reporting the test failure, so perhaps it's related.

Running the tests in Firefox and Chromium work, as does running the application in development mode and logging in manually. The problem is limited to phantomjs.

I have other acceptance tests for another route and these all pass. It seems limited to the /login route, suggesting that it is possibly related to authentication.

Debugging

I've tried debugging by adding pauseTest() to the test and "phantomjs_debug_port": 9000 to testem.js but both Firefox and Chromium do nothing when I use the debug console. This might be my lack of experience debugging phantomjs, but I would at least expect it to give me an error - it literally does nothing.

It feels as though there is a timing issue between phantomjs and something, possible ember-simple-auth, in my Ember app.

I'm not that experienced debugging phantomjs problems nor Ember acceptance test failures, so any help is appreciated.

Versions

ember-cli 2.10.0
ember-simple-auth 1.1.0
ember-cli-mirage 0.2.4