HTTPS issue only affecting some links

I’ve added the following code to functions.php so that the page’s links are automatically set as https links.

add_filter('upload_dir', function($uploads) {
    $uploads['url'] = preg_replace('#^http://#i', 'https://', $uploads['url']);
    $uploads['baseurl'] = preg_replace('#^http://#i', 'https://', $uploads['baseurl']);

    return $uploads;
});

That mostly works, but there are still some links that are still referring to the http URL. I can’t see any pattern to it. What can I do to fix the issue?