Problem using map and tracked properties

controllers.application\.js

import Controller from '@ember/controller';
import {action, set, get} from "@ember/object";
import {tracked} from "@glimmer/tracking";
import {
  TrackedObject,
  TrackedArray
} from 'tracked-built-ins';

export default class ApplicationController extends Controller {

  @tracked
	course = {
    attributes: new TrackedObject({
     	calendar: new TrackedObject({ 
        dates: new TrackedArray([
          new TrackedObject({id: "monday", attributes: new TrackedObject({ lessonId: "a"}) }),
          new TrackedObject({id: "tuesday", attributes: new TrackedObject({ lessonId: "a"}) }),
          new TrackedObject({id: "wednesday", attributes: new TrackedObject({ lessonId: "a"}) }),
          new TrackedObject({id: "thursday", attributes: new TrackedObject({ lessonId: "a"}) }),
          new TrackedObject({id: "friday", attributes: new TrackedObject({ lessonId: "a"}) }),
        ])
      })
    })
  }

  get countOfUniqueLessonIds(){
   	return this.course.attributes.calendar.dates.map(date => date.attributes.lessonId).uniq().length
  }

	@action makeMondayHaveUniqueLessonId(){
		set(this.course.attributes.calendar.dates[0].attributes, "lessonId", "b")
  }
}

templates.application\.hbs

<strong>Lesson Ids in this Course:</strong>
<ul>
{{#each this.course.attributes.calendar.dates as |date| }}
	<li>{{date.id}}: {{date.attributes.lessonId}}</li>
{{/each}}
</ul>

<p>
	<strong>Count of unique lesson ids: {{this.countOfUniqueLessonIds}}</strong>
	<br>
	Should be one and then two after clicking the button
</p>

<button {{on "click" this.makeMondayHaveUniqueLessonId}}>Make Monday have a unique lesson id</button>

twiddle\.json

{
  "version": "0.17.0",
  "EmberENV": {
    "FEATURES": {},
    "_TEMPLATE_ONLY_GLIMMER_COMPONENTS": false,
    "_APPLICATION_TEMPLATE_WRAPPER": true,
    "_JQUERY_INTEGRATION": true
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.17.2",
    "ember-template-compiler": "3.17.0",
    "ember-testing": "3.17.0"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "tracked-built-ins": "1.0.1"

  }
}