Hi @Will
If $internals is an array of posts, then the get_sub_field function is working correctly. Perhaps the issue lies in the setup_postdata funciton which can cause many problems wiht the global $post object.
Try this instead:
$internals = get_sub_field('internal_link');
foreach($internals as $p): ?>
<a href="<?php echo get_permalink( $p->ID ); ?>">
