Single Image from Field Group Not Displaying in Post

I have a field group set up within Meta Box that includes a Single Image type that I am looking to display in my Custom Post Type Post using shortcode on WordPress. Right now, everything displays as expected, except for the image. Below is the section of PHP I am using. I've also included a few reference images for context.

<div>

if ($query->have_posts()) {
        while ($query->have_posts()) {
            $query->the_post();
            $url = get_post_meta(get_the_ID(), 'url', true);
            $description = get_post_meta(get_the_ID(), 'description', true);
            $lgbtq = get_post_meta(get_the_ID(), 'lgbtq', true);
            $woman_owned = get_post_meta(get_the_ID(), 'woman_owned', true);
            $minority_owned = get_post_meta(get_the_ID(), 'minority_owned', true);
            $size_inclusive = get_post_meta(get_the_ID(), 'size_inclusive', true);
            
            $output .= '<div class="designer-item">';
            $output .= '<a href="' . esc_url($url) . '">';
            $output .= get_the_post_thumbnail(get_the_ID(), 'medium');
            $output .= '<h3>' . get_the_title() . '</h3>';
            $output .= '</a>';
            $output .= '<p>' . esc_html($description) . '</p>';
            $output .= '<ul>';
            if ($lgbtq) $output .= '<li>LGBTQ+ owned</li>';
            if ($woman_owned) $output .= '<li>Woman-owned</li>';
            if ($minority_owned) $output .= '<li>Minority-owned</li>';
            if ($size_inclusive) $output .= '<li>Size-inclusive</li>';
            $output .= '</ul>';
            $output .= '</div>';
        }
        wp_reset_postdata();
    } else {
        $output .= '<p>No designers found.</p>';
    }

    $output .= '</div>';
    return $output;

</div>