Steps to reproduce the issue

In a virgin J4.0.5 site:

• Create a Registered user.
• Create a category and give the group the user belongs to the following permissions for that category: Create, Delete, Edit State & Edit Own.
• Create a menu item of type 'Category List'.
• Login in the front end as the new user.
• Navigate to the Category List.
• Click 'New Article'.
• Enter at least the title and click 'Save'.

Expected result

The new article being succesfully created and showing up in the Category List.

Actual result

A 404 with error message: Invalid field: Start Featured

System information (as much as possible)

Joomla! 4.0.5

Additional comments

I traced the cause of the error to line 501 of the front end article model Joomla\Component\Content\Administrator\Model\ArticleModel (administrator/components/com_content/src/Model/ArticleModel.php). That line and preceeding comment read:

// Get ID of the article from input, for frontend, we use a_id while backend uses id
$articleIdFromInput = (int) $app->input->getInt('a_id') ?: $app->input->getInt('id', 0);

First of all I wonder why the back end id is checked in a front end model.
Secondly, the id obtained from $app->input can apparently contain a value other than the record id (didn't check but it looks like the category id). This sets the $articleIdFromInput to a non 0 value, whereas it should be 0 for new articles. The result is that permission, to edit certain attributes of the the record, are checked against a wrong or invalid record id, eventually resulting in the above mentioned error or unjustified acceptation or rejection of the record.

EDIT: I removed the proposed solution. Thanx to @ReLater who pointed out my stupid mistake.

Maybe this is a better approach:

$articleIdFromInput = $app->isClient('site')
			? (int) $app->input->getInt('a_id', 0) 
			: $app->input->getInt('id', 0);