ED Embedded records bug

adapters.bar\.js

import RESTAdapter from '@ember-data/adapter/rest';

export default class extends RESTAdapter {}

adapters.foo\.js

import RESTAdapter from '@ember-data/adapter/rest';

export default class extends RESTAdapter {
  findRecord() {
    return {
      foo: {
        id: 1,
        name: 'Foo 1',
        bars: [{
          id: 1,
          name: 'Bar 1'
        }]
      }
    }
  } 
  
  updateRecord() {
    return {
      foo: {
        id: 1,
        name: 'Foo 1 (saved)',
        bars: [{
          id: 1,
          name: 'Bar 1 (saved)'
        }]
      }
    }
  }
}

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object'
import { inject } from '@ember/service';

export default class ApplicationController extends Controller {
  @inject store;
  
  @action
  update() {
    this.foo.set('name', 'Foo 1 (edited)');
    this.foo.bars.objectAt(0).set('name', 'Bar 1 (edited)');
  }
  
  @action
  async save() {
    await this.foo.save(); 
    
    console.log(
      this.foo.get('currentState.stateName')
    );
 
    console.log(
      this.foo.bars.objectAt(0)
        .get('currentState.stateName')
    );
  }
}

models.bar\.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';

export default class extends Model {
  @attr() name;
}

models.foo\.js

import Model from 'ember-data/model';
import attr from 'ember-data/attr';
import {hasMany } from 'ember-data/relationships';

export default class extends Model {
  @attr() name;
  @hasMany('bar', { async: false }) bars;
}

routes.application\.js

import Route from '@ember/routing/route';
import { inject } from '@ember/service';

export default class extends Route {
  @inject store;
  
  model() {
    return this.store.findRecord('foo', 1); 
  }
  
  setupController(controller, model) {
    controller.set('foo', model);
  }
}

serializers.application\.js

import RESTSerializer from '@ember-data/serializer/rest';

export default class extends RESTSerializer {}

serializers.bar\.js

import RESTSerializer from '@ember-data/serializer/rest';

export default class extends RESTSerializer {}

serializers.foo\.js

import RESTSerializer, { EmbeddedRecordsMixin } from '@ember-data/serializer/rest';

export default class extends RESTSerializer.extend(EmbeddedRecordsMixin) {
  attrs = {
    bars: {
      embedded: 'always'
    }
  };
}

templates.application\.hbs

<h1>ED Bug?</h1>

<p>One Foo has many Bars</p>

<p>This demo has 1 Foo, which has 1 Bar (as an embedded record)</p>

<p>Pressing Update sets Foo 1's name to <em>'edited'</em></p>

<p>Pressing Save makes the server respond with <em>'saved'</em></p>

<p>Foo is saved correctly, but the embedded records on Foo (the Bars) are not updated.</p>

<hr>

<code>foo.name</code>: {{this.foo.name}}

<br><br>

<code>foo.bars</code>: <br>

{{#each this.foo.bars as |bar|}}
  {{bar.name}}<br>
{{/each}}

<br><br><br>

<button {{on "click" this.update}}>Update</button>
<button {{on "click" this.save}}>Save</button>

twiddle\.json

{
  "version": "0.17.1",
  "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/3.5.1/jquery.js",
    "ember": "3.18.1",
    "ember-template-compiler": "3.18.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-data": "3.18.0"
  }
}