Ember - No Data in hasMany Relationship On Initial Load

ember-cli - 3.20, ember-data - 3.30

I am trying to modify the data in a hasMany relationship in the controller setup but the relationship has no data. However, all the data is there after the page is fully loaded (i.e. in my template/actions, all relationship data is there)

I have a Quiz application with Many-Many relationship with Questions.

models/Quiz.js

import { computed } from '@ember/object';
import DS from 'ember-data';

const { attr, hasMany, Model } = DS;

export default Model.extend({
  description: attr('string'),
  questions: hasMany('question', {async: true}) //also tried with async false

});

models/Question.js

export default Model.extend({
  question: attr('string'),
  quizzes: hasMany('quiz', {async: true}) //also tried with async false
});

Go to url '/quiz/1' and Route calls findRecord on quiz

routes/quizzes/quiz.js

import Route from '@ember/routing/route';

export default Route.extend({
  model(params) { return this.store.findRecord('quiz', params.quiz_id); }
});

controllers/quizzes/quiz.js

import { computed } from '@ember/object';
import Controller from '@ember/controller';

export default Controller.extend({
  quiz: computed.alias('model'),

  modelChanged: function() {
    let quiz = this.get('quiz');
    let questions = quiz.get('questions'); //questions has no data
    questions.then(questions => {
      Promise.all(questions.map(question => {
        //modify questions/answers here
      }));
    });
  }.observes('model')

actions: {
  getQuestions() {
    let questions = this.get('quiz.questions');  //questions now has data
  }
})};

I have tried to get the question data in both setupController() and afterModel() with no luck.

What has worked:
The quizzes are nested routes able to select between each quiz to display. So if you navigate from '/quiz/1' to '/quiz/2' and then back to 'quiz/1', the question data is available in the observer, setupController, afterModel, etc. So, the second time you access a specific quiz, the data is available in setup. (data is always available in template/actions).

Any ideas?