[Bug] Returning `transitionTo` in a `async beforeModel` hook prevents the app from booting.

[knownasilya]

🐞 Describe the Bug

Returning transitionTo or router.transitionTo in a async beforeModel hook prevents the app from booting.

🔬 Minimal Reproduction

https://github.com/knownasilya/ember-async-before-model-repro

Also happens with model, and afterModel. Works fine with redirect.

🌍 Environment

• Ember: - 3.26 and 3.27
• Node.js/npm: - 12.22.1/yarn 1.22.4
• OS: - macOS (latest)
• Browser: - Chrome (latest)

[knownasilya]

Btw something like this also causes the issue

beforeModel() {
    // waitForAuthentication returns an RSVP Promise, if I change to native promise it works fine
    return this.session.waitForAuthentication().then((isAuthenticated) => {
      if (!isAuthenticated) {
        return this.router.transitionTo('setup');
      }

      return;
    });
  }

but not this:

  beforeModel() {
    return this.router.transitionTo('setup');
  }

Seems like the issue is related to native vs RSVP promise result.

[knownasilya]

Ok, seems like it's not rsvp related and the above scenario doesn't work if not using rsvp promise.
But If i put the transitionTo into a next, then it works. Something about transitioning inside another promise, wether that transition is returned or not causes the issue.