broken relationship

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
  init() {
		this.set('parent', this.store.createRecord('parent', {name:'foo'}))
  }, 
  
  actions: {
    addNewChild() { 
      console.log(`@ foo`)
      const child = this.store.createRecord('child', {parent: this.parent})
      this.set('child', child)
    },
    
    unloadLastChild() {
      if (this.child) { 
        // Commented out and it is broken somehow
        // this.child.set('parent', null)
        this.child.unloadRecord()
        this.set('child', null)
      }
    }
  }
});

models.child.js

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

export default Model.extend({
  parent: belongsTo('parent')
});

models.parent.js

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

export default Model.extend({
  children: hasMany('child'),
  name: attr('string')
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{parent.children.length}}

<button onclick={{action "addNewChild"}}>Add</button>
<button onclick={{action "unloadLastChild"}}>Unload last child</button>
<br>
<br>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}