wp_update_attachment_metadata always return false

Question

I am using the following code to insert an image as a post attachment. The code works OK, except that wp_update_attachment_metadata will always return false. Based on the document https://developer.wordpress.org/reference/functions/wp_update_attachment_metadata/, false indicate the post is invalid. However, I check the media library and can see the image is added properly with all the required info, also different dimension versions are generated in the upload dir, so I cannot finger out why wp_update_attachment_metadata returns false.

<?php 
// $filename should be the path to a file in the upload directory.
$filename = '/path/to/uploads/2013/03/filename.jpg';

// The ID of the post this attachment is for.
$parent_post_id = 37;

// Check the type of file. We'll use this as the 'post_mime_type'.
$filetype = wp_check_filetype( basename( $filename ), null );

// Get the path to the upload directory.
$wp_upload_dir = wp_upload_dir();

// Prepare an array of post data for the attachment.
$attachment = array(
    'guid'           => $wp_upload_dir['url'] . '/' . basename( $filename ), 
    'post_mime_type' => $filetype['type'],
    'post_title'     => preg_replace( '/\.[^.]+$/', '', basename( $filename ) ),
    'post_content'   => '',
    'post_status'    => 'inherit'
);

// Insert the attachment.
$attach_id = wp_insert_attachment( $attachment, $filename, $parent_post_id );

// Make sure that this file is included, as wp_generate_attachment_metadata() depends on it.
require_once( ABSPATH . 'wp-admin/includes/image.php' );

// Generate the metadata for the attachment, and update the database record.
$attach_data = wp_generate_attachment_metadata( $attach_id, $filename );
// $result is always false
$result = wp_update_attachment_metadata( $attach_id, $attach_data );

set_post_thumbnail( $parent_post_id, $attach_id );
?>