Textarea Bug?

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  
  textValue: 'Some random text value',
  
  actions: {
    addText() {
     	let textValue = this.get("textValue");
      textValue += "[SOME MORE TEXT]";
      this.set("textValue", textValue);
    }
  }
  
});

templates.application.hbs

Text Value:<br />
{{textValue}}
<br /><br />

<button type="button" {{action "addText"}}>Add Text</button>
<br /><br />

<textarea onkeyup={{action (mut textValue) value="target.value"}} style="width:100%">{{textValue}}</textarea>

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}