Reproduce bug

Clicking button updates Ember Data model attribute `num`. The tracking
for the getter `title` works on first update, but subsequently fails
to update the template.

If instead of using `@attr` we use `@tracked`, clicking the button
any number of times will always update the template.

app/controllers/application.js

@@ -0,0 +1,11 @@
+import Controller from '@ember/controller';
+import { action } from '@ember/object';
+
+export default class ApplicationController extends Controller {
+
+  @action
+  changeTitle() {
+    this.model.num = Math.random();
+  }
+
+}

app/models/post.js

@@ -0,0 +1,18 @@
+import DS from 'ember-data';
+// import { tracked } from '@glimmer/tracking';
+
+const { Model, attr } = DS;
+
+export default class Post extends Model {
+
+  // This doesn't work after the first property change:
+  @attr() num;
+
+  // This works:
+  // @tracked num;
+
+  get title() {
+    return `Post ${this.num}`;
+  }
+
+}

app/routes/application.js

@@ -0,0 +1,9 @@
+import Route from '@ember/routing/route';
+
+export default class ApplicationRoute extends Route {
+
+  model() {
+    return this.store.createRecord('post');
+  }
+
+}

app/templates/application.hbs

@@ -1,5 +1,3 @@
-{{!-- The following component displays Ember's default welcome message. --}}
-<WelcomePage />
-{{!-- Feel free to remove this! --}}
+<h1>Title: {{this.model.title}}</h1>
 
-{{outlet}}
+<button {{action "changeTitle"}}>Click me a few times</button>