WordPress at a glance
action is not described

block_editor_meta_box_hidden_fields action-hook . WP 5.0.0

Add hidden input fields to the meta box save form.

Hook into this action to print <input type="hidden" ... /> fields, which will be POSTed back to the server when meta boxes are saved.

Usage

add_action( 'block_editor_meta_box_hidden_fields', 'action_function_name_4366' );
function action_function_name_4366( $post ){
	// action...
}
$post
-

Where the hook is called

the_block_editor_meta_box_post_form_hidden_fields()
wp-admin/includes/post.php 2275
do_action( 'block_editor_meta_box_hidden_fields', $post );

Where the hook is used (in WP core)

Does not used.