我认为这是一种更好、更干净的方法，可以实现你想要实现的目标。

// get_adjacent_post will return $post object
$prev_post = get_adjacent_post(true,\'\',true);
//Using get_the_post_thumbnail() to get featured image of previous post
$prev_thumbnail = get_the_post_thumbnail( $prev_post->ID, array( 100, 100));

$next_post = get_adjacent_post(true,\'\',false);
//Using get_the_post_thumbnail() to get featured image of next post
$next_thumbnail = get_the_post_thumbnail( $next_post->ID, array( 100, 100));

the_post_navigation(
    array(
        \'in_same_term\' => true,
        \'next_text\' => \'<p class="meta-nav">\'. $next_thumbnail .\'Next Post &rarr;</p><p class="post-title">%title</p>\',
        \'prev_text\' => \'<p class="meta-nav">\'. $prev_thumbnail .\'&larr; Previous Post</p><p class="post-title">%title</p>\',
    )
);

好的，上述解决方案将需要额外的next\\u post\\u link筛选器和prev\\u post\\u link筛选器，并覆盖\\u post\\u导航模板，这不是一个真正好的解决方案。因此，根据您的代码，尝试一下这个。我已经测试了同一类别的3篇帖子，其循环没有问题。

$prev_post = get_adjacent_post(true,\'\',true);
if($prev_post):
    $prev_post_id = $prev_post->ID;
    $prev_thumbnail = get_the_post_thumbnail($prev_post_id, array( 100, 100));
else:
    $abjacent_categories = get_the_category(); 
    $ids = array();
    foreach($abjacent_categories as $category) {
        $ids[] = $category->term_id;
    }
    $last = new WP_Query( array( 
        \'post_type\' => \'post\',
        \'posts_per_page\' => 1, 
        \'orderby\'         => \'post_date\',
        \'order\'           => \'DESC\',
        \'category__in\' => $ids, 
/* use category__and if you want post to be in all categories as current post. 
For now its happy with one of current post categories. 
Depending on this will give different results at the end so keep that in mind. */
     ) ); 
    $last->the_post(); 
    $prev_post_id = get_the_ID();
    $prev_thumbnail = get_the_post_thumbnail( $prev_post_id, array( 100, 100));
wp_reset_query();
endif;

$next_post = get_adjacent_post(true,\'\',false);
if($next_post):
    $next_post_id = $next_post->ID;
    $next_thumbnail = get_the_post_thumbnail($next_post_id, array( 100, 100));
else:
    $abjacent_categories = get_the_category(); 
    $ids = array();
    foreach($abjacent_categories as $category) {
        $ids[] = $category->term_id;
    }
    $first = new WP_Query( array( 
        \'post_type\' => \'post\',
        \'posts_per_page\' => 1, 
        \'orderby\'         => \'post_date\',
        \'order\'           => \'ASC\',
        \'category__in\' => $ids,     
/* use category__and if you want post to be in all categories as current post. 
For now its happy with one of current post categories. 
Depending on this will give different results at the end so keep that in mind. */
     ) ); 
    $first->the_post(); 
    $next_post_id = get_the_ID();
    $next_thumbnail = get_the_post_thumbnail( $next_post_id, array( 100, 100));
wp_reset_query();
endif;

对于html，可以使用以下模板进行测试

<div class="nav-links">
    <div class="nav-previous">
        <a href="<?php echo get_the_permalink($prev_post_id); ?>" rel="prev">
            <p class="meta-nav"><?php echo $prev_thumbnail ?> &larr; Previous Post</p><p class="post-title"><?php echo get_the_title($prev_post_id); ?></p>
        </a>
    </div>
    <div class="nav-next">
        <a href="<?php echo get_the_permalink($next_post_id); ?>" rel="next">
            <p class="meta-nav"><?php echo $next_thumbnail ?> Next Post &rarr;</p><p class="post-title"><?php echo get_the_title($next_post_id); ?></p>
        </a>
    </div>
</div>