FIXED! A friend help me. Here’s an update.
<?php if( $ids = get_field('featured_posts', 'options', false, false) ) :
$query = new WP_Query(array(
'post_type' => 'post',
'posts_per_page' => 5,
'post__in' => $ids,
'post_status' => 'publish',
));
if ( $query->have_posts() ) : ?>
<div class="">
<ul class="slides">
<?php while ( $query->have_posts() ) : $query->the_post(); // variable must NOT be called $post (IMPORTANT) ?>
<li>Loop here</li>
<?php endwhile; wp_reset_postdata(); ?>
</ul>
</div>
<div class="fix"></div>
<?php else : ?>
<?php endif; endif; ?>
