Problem with wp_insert_post

According to the Codex, when using wp_insert_post a page is created or updated depending upon the contents of $post_id. If this is zero, a new page is created. Otherwise, the post with the id which is set in $post_id will be updated.

I have a routine which creates a page using wp_insert_post. The routine works. However, if I try to update an already created new page, a second page is created rather than the old one being modified – even though the routine has a mechanism to avoid this:

$list_name = $_REQUEST["list_name"];

// Set (or get) the page id so as to create or update	
$page_id = 0;	
global $wpdb; // Code is within a function
	
$sql = $wpdb->prepare( "SELECT ID, post_title FROM wp_mgf_posts WHERE post_title = %s ORDER BY ID DESC LIMIT 1",$list_name);
$results = $wpdb->get_results( $sql,ARRAY_A);
	
var_dump($results); 	
	
foreach($results as $result)
{
$page_id = $result['ID'];	
}
	
echo 'Page id = ' . $page_id;
	
// Create (or update) the page	
$plant_list = array(
  'post_ID'      => $page_id,     // 0 for create, existing page id for update
  'post_title'    => $list_name,  // the title of the list
  'post_content'  => $content,   // page content
  'post_status'   => 'publish',
  'post_author'   => 1,
  'post_type'   => 'page',
  'post_category' => array(26,30)  // development and plant list categories
);
$insert = wp_insert_post( $plant_list );

As you can see, I have put in a var_dump and an echo of the retrieved post_id. This is the result:

array(1) {
[0]=>
array(2) {
[“ID”]=>
string(4) “7001”
[“post_title”]=>
string(18) “Late Summer Colour”
}
}
Page id = 7001

This looks OK to me. Presumably, I’m doing something wrong but have no idea what. Can anyone help?