(action (mut foo)) vs (fn (mut foo))

controllers.application\.js

import Controller from '@ember/controller';

import { tracked } from '@glimmer/tracking';

export default class ApplicationController extends Controller {
  @tracked
  prop;
  
  @tracked
  propDidUpdate = false;
}

templates.application\.hbs

<Input @type="text" @value={{this.prop}} placeholder="Edit property value" />

<div {{did-update (action (mut this.propDidUpdate) true) this.prop}}>
  Property did update? <Input @type="checkbox" @checked={{this.propDidUpdate}} />
</div>

twiddle\.json

{
  "version": "0.16.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.14.3",
    "ember-template-compiler": "3.14.3",
    "ember-testing": "3.14.3"
  },
  "addons": {
    "@ember/render-modifiers": "1.0.2"
  }
}