set_post_thumbnail when user is not logged in

I’m currently using the set_post_thumbnail() function to insert a programmatically uploaded image to a post.
The problem is that the function works perfectly when the user is logged in only.

If the user is not logged in, in the back-end, wordpress says that the file is attached to the post but does not display it as a featured image.
I don’t have any conditional statement in that function.

Does anybody have had the same issue?
Is that a normal behavior?

Edit: Noticed that the issue showed up since I’ve upgraded WordPress from 3.5.1 to 3.5.2 but I really don’t know if it’s related

Edit: Here is a sample of my code

// This part is for testing purpose, the file is well placed with the right permission
$dir = '/home/web/tmpdir';
$data = $_POST['imageCanvas'];
$preview = base64_decode( str_replace( 'data:image/png;base64,', '', $data ) );
$file = '000011.png'; 
$filename = $dir . '/' . $file;
file_put_contents( $filename, $preview );

// Here I copy the test file directly into the wordpress uploads dir
// After this line I find the file in the uploads dir, so, let's say that for now it works as intended
$wp_filetype = wp_check_filetype( basename( $file ), null );
copy( $filename, '/home/web/wordpress/wp-content/uploads/' . basename( $file ) );
$attachment = array(
    "post_mime_type" => $wp_filetype['type'],
    "post_title" => preg_replace( '/.[^.]+$/', '', basename( $file ) ),
    "post_content" => '',
    "post_status" => 'inherit'
);

// This part worked well until I recently upgraded WP to 3.5.2...
$attach_id = wp_insert_attachment( $attachment, $file, $lastpostID );
require_once( ABSPATH . 'wp-admin/includes/image.php' );
$attach_data = wp_generate_attachment_metadata( $attach_id, $file );
wp_update_attachment_metadata( $attach_id, $attach_data );
set_post_thumbnail( $lastpostID, $attach_id );

// This part returns the right ids in Apache logs...
error_log( 'attachid: ' . $attach_id . ' - postid: ' . $lastpostID );

I can find the file attached to his post into the wordpress media library:

But if I open the post, I don’t see any featured image.