Please enable debug mode and then post the resultant stack trace

Call stack
1 | () | JROOT/libraries/vendor/joomla/utilities/src/ArrayHelper.php:72
2 | Joomla\Utilities\ArrayHelper::toObject() | JROOT/libraries/src/MVC/Model/AdminModel.php:930
3 | Joomla\CMS\MVC\Model\AdminModel->getItem() | JROOT/administrator/components/com_content/Model/ArticleModel.php:394
4 | Joomla\Component\Content\Administrator\Model\ArticleModel->getItem() | JROOT/administrator/components/com_content/Model/ArticleModel.php:547
5 | Joomla\Component\Content\Administrator\Model\ArticleModel->loadFormData() | JROOT/libraries/src/MVC/Model/FormModel.php:272
6 | Joomla\CMS\MVC\Model\FormModel->loadForm() | JROOT/administrator/components/com_content/Model/ArticleModel.php:455
7 | Joomla\Component\Content\Administrator\Model\ArticleModel->getForm() | JROOT/libraries/src/MVC/View/AbstractView.php:146
8 | Joomla\CMS\MVC\View\AbstractView->get() | JROOT/administrator/components/com_content/View/Article/HtmlView.php:81
9 | Joomla\Component\Content\Administrator\View\Article\HtmlView->display() | JROOT/libraries/src/MVC/Controller/BaseController.php:679
10 | Joomla\CMS\MVC\Controller\BaseController->display() | JROOT/administrator/components/com_content/Controller/DisplayController.php:59
11 | Joomla\Component\Content\Administrator\Controller\DisplayController->display() | JROOT/libraries/src/MVC/Controller/BaseController.php:717
12 | Joomla\CMS\MVC\Controller\BaseController->execute() | JROOT/libraries/src/Dispatcher/Dispatcher.php:162
13 | Joomla\CMS\Dispatcher\Dispatcher->dispatch() | JROOT/libraries/src/Component/ComponentHelper.php:354
14 | Joomla\CMS\Component\ComponentHelper::renderComponent() | JROOT/libraries/src/Application/AdministratorApplication.php:111
15 | Joomla\CMS\Application\AdministratorApplication->dispatch() | JROOT/libraries/src/Application/AdministratorApplication.php:154
16 | Joomla\CMS\Application\AdministratorApplication->doExecute() | JROOT/libraries/src/Application/CMSApplication.php:243
17 | Joomla\CMS\Application\CMSApplication->execute() | JROOT/administrator/includes/app.php:38
18 | require_once() | JROOT/administrator/index.php:36

Can you try changing this line

to
$item = ArrayHelper::toObject($properties, 'JObject');

Chance Line as suggested works, Article is created/loaded as expected.

The CMSObject should be changed to Joomla\CMS\Object\CMSObject. @brianteeman you want to do the PR or shall I?

@roland-d can you do it please - not got time this morning

You can also write CMSObject::class.

@laoneo That doesn't work for me. Did you test this?

The PR I created uses the fully qualified name: #21300

I will close this as we have a PR.

Is not CMSObject deprecated at 4.0?

It is, but we have so many dependencies that it is basically impossible to do it without a hard BC break.