#18969 bug?

components.outer\.js

import Component from "@glimmer/component";
import { tracked } from "@glimmer/tracking";
import { action } from "@ember/object";

let id = 0;

export default class extends Component {
  @tracked items = [];

	@action
	add() {
   this.items.push({ id: id++, value: 'hello'})
   this.items = this.items;
  }

	@action
	remove() {
    this.items.pop();
   	this.items = this.items;
  }

  @action
  clearItems() {
    this.items = [];
  }
}

helpers.map-by\.js

import { helper } from "@ember/component/helper";

export default helper(function mapBy([array, propertyName]) {
  console.log("C", array);
  return array.mapBy(propertyName);
});

templates.application\.hbs

<div style="padding: 4px; margin: 4px; background-color: lightyellow;">
	Trying to reproduce https://github.com/emberjs/ember.js/issues/18969
</div>

<Outer />

templates.components.inner\.hbs

<h1>List</h1>

There are {{@items.length}} items:

{{log 'A' @items}}
{{#if @items}}
  {{log 'B' @items}}
  {{#let (map-by @items 'id') as |itemIds|}}
    itemIds: {{itemIds}}
  {{/let}}
{{/if}}

templates.components.outer\.hbs

<button {{on "click" this.add}}>Add</button>
<button {{on "click" this.remove}}>Remove</button>
<button {{on "click" this.clearItems}}>Clear</button>

<Inner @items={{this.items}} />

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