HTML output from WordPress shortcode is wrapped in unexpected tags

Please try as below:

 function wp_reservations_shortcode($atts) {
    // remove auto paragraphs for shortcode content
    remove_filter('the_content', 'wpautop');
    remove_filter('the_excerpt', 'wpautop');

    ob_start();
    ?>
    <section>
        <section class="calendar-header">
            <button id="prev-month">Prev</button>
            <div class="current-date">July 2024</div>
            <button id="next-month">Next</button>
        </section>
        <section class="calendar-body">
            <div class="calendar-weekdays"></div>
            <div class="calendar-days"></div>
        </section>
    </section>
    <?php
    $html = ob_get_clean();

    // Re-enable automatic paragraphs
    add_filter('the_content', 'wpautop');
    add_filter('the_excerpt', 'wpautop');

    return $html;
}
add_shortcode('wp_reservations', 'wp_reservations_shortcode');