New Twiddle

controllers.application\.js

import Controller from '@ember/controller';
import { alias } from '@ember/object/computed';
import EmberObject,  { computed } from '@ember/object';
import { A } from '@ember/array';

const AttributeModel = EmberObject.extend({
  countAdditives: alias('additives.length'),
  additives: A()
});

const RootModel = EmberObject.extend({
	allAdditives: computed('[email protected]', function(){
  	return this.metaAttributes.toArray().reduce((acc, el)=>{
    	return acc.concat(el.additives.toArray());
    }, []);
  }),
  metaAttributes: A([AttributeModel.create()])
});

export default class ApplicationController extends Controller {
  version = Ember.VERSION;
	constructor() {
  	super(...arguments);
    let model = RootModel.create();
    this.set('valueOne', model.allAdditives.slice().length);
    model.metaAttributes[0].additives.pushObject('foo');
    this.set('valueTwo', model.allAdditives.slice().length);
  }
	
}

templates.application\.hbs

Simplier reproduction, (current ember {{version}}):
<br><br>
Case1: {{valueOne}} / should be 0<br>
Case2: {{valueTwo}} / should be 1<br>
<br><br>
Change <b>ember-source</b> from 3.12.1 to 3.13.1+ to see regression.

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": "3.12.1",
    "ember-template-compiler": "3.12.1",
    "ember-testing": "3.18.1"
  },
  "addons": {
    "@glimmer/component": "1.0.0"
  }
}