get_adjacent_post: Strange Behavior!

I’m putting together an archive of posts in a custom Post Type that are arranged sequentially both by Title and ID. My plan was to use get_adjacent_post to link to previous and next posts for a smooth browsing experience, but in practice it’s producing wildly unexpected results and I can’t figure out why.

Here’s an example: “0099” has an ID of 104, while “0098” has an ID of 103 and “0100” has an ID of 105. But in this case, $previous is generating a link to “0072” (ID 77) and $next is generating an error (visible in Debug Mode), as it does on every post.

Finally, here’s the preliminary template code:

<?php get_header(); ?>

<!-- single-phase-one.php -->

<main>

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

<article>
<h1><?php the_title(); ?></h1>
<?php the_content(); ?>

<img style="width: 100%;" src="<?php bloginfo('url') ?>/comics/westward_<?php the_title(); ?>.jpg" />

<?php $previous = get_adjacent_post(); ?>
<a href="<?php bloginfo('wpurl'); echo '/phase-one-comics/'; echo $previous->post_title; ?>">&larr; <?php echo $previous->post_title; ?> </a>

&nbsp; &nbsp; &nbsp; 

<?php $next = get_adjacent_post(false,'',true,''); ?>
<a href="<?php bloginfo('wpurl'); echo '/phase-one-comics/'; echo $next->post_title; ?>"><?php echo $next->post_title; ?> &rarr;</a>

</article>

<?php endwhile; else: ?>
<p><?php _e('Sorry, no posts matched your criteria.'); ?></p>
<?php endif; ?>

</main>

<?php get_footer(); ?>

Thanks!