Component Subexpressions one-way by default?

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  valueList: [1, 2, 3, 4],
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{#each valueList as |v|}}
  {{component 'x-thing' val=(mut v)}}
  Value outside: {{v}}
  <br>
  <br>
{{/each}}

twiddle.json

{
  "version": "0.6.5",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.4.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.4.3/ember-template-compiler.js"
  }
}

x-thing.template.hbs

<button {{action (action (mut val) 10)}}>
  Change to 10
</button>
Val in component: {{val}}