Bug Demo

controllers.application\.js

import Controller from '@ember/controller';
import EmberObject, { get, set } from '@ember/object';
import { getOwner } from '@ember/application';

const RecordMeta = new WeakMap();
function getRecordMeta(record) {
   let meta = RecordMeta.get(record);
    if (meta === undefined) {
     meta = Object.create(null);
     RecordMeta.set(record, meta);
   }
   return meta;
 }

class Model extends EmberObject {
  get isError() {
     let meta = getRecordMeta(this);
     let isError = get(meta, 'isError');
     return isError !== undefined ? isError : false;
  }
  set isError(v) {
    let meta = getRecordMeta(this);
    set(meta, 'isError', v);}
}

export default class ApplicationController extends Controller {
  appName = 'Ember Twiddle';

  get records() {
    let owner = getOwner(this);
    owner.register('model:user', Model);
     let factory = owner.factoryFor(`model:user`);
     let record = factory.create({ name: 'Chris', isError: true });
     
    return [record];
  }
}

templates.application\.hbs

<h1>Welcome to {{this.appName}}</h1>
<br>
<ul>
{{#each this.records as |record|}}
  <li>
    {{record.name}}: {{if record.isError 'isError' 'not isError'}}
  </li>
{{/each}}
</ul>
<br>
{{outlet}}
<br>
<br>

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"
  }
}