Unload record error

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle'
});

controllers.index.js

import Ember from 'ember';

export default Ember.Controller.extend({
  mainGroup: Ember.computed('model', function() {
    return this.model.filter(function(group){
    	return group.group === null;
    });
  }),
  
  actions: {
    onCreateGroup: function(item) {
      const newGroup = this.store.createRecord('group');
      newGroup.setProperties({
        name: 'New Group',
        group: item.group
      });
      
      item.set('group', newGroup);
    },
    
    onUnloadItem: function(item) {
      item.unloadRecord();
    }
  }
});

mirage.config.js

export default function() {
  this.get('/groups', function(){
    return {
      data: [
        {
          id: '1',
          type: 'groups', 
          attributes: {
            name: 'Main Group'
          },
          relationships: {
            items: {
            	data: [
                { type: 'items', id: 'item-1' },
                { type: 'items', id: 'item-2' },
                { type: 'items', id: 'item-3' },
              ]
            }
          }
        }
      ],
      included: [
      	{
        	id: 'item-1',
          type: 'items',
          attributes: { name: 'Item 1' }
        },
        {
        	id: 'item-2',
          type: 'items',
          attributes: { name: 'Item 2' }
        },
        {
        	id: 'item-3',
          type: 'items',
          attributes: { name: 'Item 3' }
        }
      ]
    }
  }) 
};

mirage.serializers.application.js

import { JSONAPISerializer } from 'ember-cli-mirage';

export default JSONAPISerializer;

models.group.js

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

export default Model.extend({
	name: attr('string'),
  
  group: belongsTo('group', { async: false, inverse: 'groups' }),
  
  items: hasMany('item', { async: false }),
  groups: hasMany('group', { async: false, inverse: 'group' })
});

models.item.js

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

export default Model.extend({
  name: attr('string'),
  
  group: belongsTo('group', { async: false })
});

routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
  model: function() {
    return this.store.findAll('group');
  }
});

templates.application.hbs

{{outlet}}

<h1>Unload Record Error</h1>

<h3>Model setup:</h3>
<ol>
	<li>
  	Group
    <ul>
    	<li>belongs to group</li>
    	<li>has many groups</li>
      <li>has many items</li>
     </ul>
  </li>
  <li>
  	Item
    <ul>
    	<li>belongs to group</li>
     </ul>
  </li>
</ol>

<h3>Steps to replicate:</h3>
<ol>
  <li>
    Click "Create New Group" for Item 1. This will create a sub-group under Group 1 named New Group. Item 1 will now be a child of New Group.
   </li>
  <li>
    Click "Unload Item" for Item 3. Notice that Item 1 is displayed twice, under both Group 1 and New Group.
   </li>
 </ol>
 
Note that this bug can be reproduced for any Item under Group 1.

templates.index.hbs

<ul>
  {{#each mainGroup as |group|}}
    <li>
    	{{group.name}}
      <ol>
      	{{#each group.groups as |subGroup|}}
        	<li>
          	{{subGroup.name}}
            <ol>
              {{#each subGroup.items as |item|}}
                <li>
                  {{item.name}}
                </li>
              {{/each}}
            </ol>
          </li>
        {{/each}}
      	{{#each group.items as |item|}}
        	<li>
          	{{item.name}}
            <button type="button" onclick={{action (action 'onCreateGroup' item)}}>Create New Group</button>
            <button type="button" onclick={{action (action 'onUnloadItem' item)}}>Unload Item</button>
          </li>
        {{/each}}
      </ol>
    </li>
  {{/each}}
</ul>

twiddle.json

{
  "version": "0.15.1",
  "ENV": {
    "ember-cli-mirage": {
      "enabled": true
    }
  },
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.13.2",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.13.1",
    "ember-cli-mirage": "0.4.14"
  }
}