WordPress add_action('wp_head'... not working

Question

I'm trying to preload an image by adding a <link rel="preload"... to the site head, with the following code:

if (is_front_page()) {
    add_action('wp_head', function(){ ?>
        <link rel="preload" fetchpriority="high" as="image" href="/wp-content/uploads/2021/12/blog-bg-1536x907.jpeg" type="image/webp">
    <?php });
}

I've added this using the Code Snippets plugin. The snippet's active (on the front-end only) but it's not working. There's no errors in the debug log, I've tried changing the priority and even tried commenting out the is_front_page() conditional but nothing seems to work. Any ideas?