New Twiddle

components.outer-component\.js

import Component from '@ember/component';
import { action } from '@ember/object';

export default class extends Component {
  init() {
    super.init();
    this.setShowYield(this.showYield);
  }
 
  @action
  setShowYield(value) {
    this.set('internalShowYield', value);
  }
}

controllers.application\.js

import Controller from '@ember/controller';
import { action } from '@ember/object';

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

  time = new Date();
  
  @action
  updateTime() {
    this.set('time', new Date());
  }
}

templates.application\.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
<hr>
{{#outer-component time=time showYield=false}}
<p>This is yielded</p>
{{/outer-component}}
<hr>
<br>
<button {{action 'updateTime'}}>Update Time</button>
<br><br>
	Repro Step:
  <ol>
  	<li>Click Show Yield to set showYield to true</li>
  	<li>Click Update time which updates another attribute on the component</li>
  	<li>Notice that showYield is reset back to false</li>
	</ol>

templates.components.outer-component\.hbs

Time: {{this.time}}

{{#if this.internalShowYield}}
	{{yield}}
{{else}}
	<p>You are not showing yield</p>
{{/if}}
<button {{action this.setShowYield true}}>Show Yield</button>

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": "3.12.2",
    "ember-template-compiler": "3.12.2",
    "ember-testing": "3.12.2"
  },
  "addons": {
    "@glimmer/component": "1.0.0",
    "ember-data": "3.12.5"
  }
}