I have figured this out. The solution is as below, with the user field set to return “User Array”:
<?php $author_id = get_field('user')['ID']; ?>
by <a href="<?php echo get_author_posts_url($author_id) ?>" rel="author"><?php echo get_field('user')['display_name']; ?></a>
Credit to @imvrk at this thread for the steer: https://support.advancedcustomfields.com/forums/topic/display-multiple-selected-users-not-working/
