Oh, you’ll probably want to put the query for the attached posts inside the loop and pass the ID. Maybe something like:
<?php
$loop = new WP_Query( array( 'post_type' => 'clients', 'posts_per_page' => 10 ) );
?>
<?php if($loop->have_posts()) : ?>
<?php while ( $loop->have_posts() ) : $loop->the_post(); ?>
<div class="postbox-container postbox">
<h3><?php the_title(); ?></h3>
<?php
$ids = get_field('attach_to_client', $loop->ID);
$query = new WP_Query(array(
'post_type' => array('research', 'project_brief','persona','task_models', 'user_stories', 'style_guide'),
'posts_per_page' => -1,
'post__in' => $ids,
'post_status' => 'any',
'orderby' => 'modified',
));
?>
<?php if ($query->have_posts()) : ?>
<ul>
<?php
while($query->have_posts()) :
$query->the_post();
?>
<li><?php the_title(); ?></li>
<?php endwhile; ?>
</ul>
<?php else: ?>
<p>Oops, there are no posts.</p>
<?php endif; ?>
</div>
<?php endwhile; endif; wp_reset_query(); ?>
