pre_get_posts query order not the same

I have a custom post-type called “scholars” and I’m sorting the archive with the pre_get_posts hook, like so:

add_action('pre_get_posts', 'my_queries');
function my_queries($query)
{
  if (!$query->is_main_query()) return;

  if ($query->is_post_type_archive('scholars')) {
    $query->set('posts_per_page', -1);
    $query->set('meta_key', 'year');
    $query->set('orderby', 'meta_value_num');
  }
}

It works just fine in the archive page, posts are ordered by year but then I use the plugin in the single page and the order is not the same, I can’t even tell what the order is TBH, is not by date, is not by custom meta apparently. What could be the problem?

<nav class="scholars-navigation">
  <span class="prev"><?php previous_post_link_plus('meta_key=year&order_by=numeric') ?></span>
  <h2><?php the_title() ?></h2>
  <span class="next"><?php next_post_link_plus('meta_key=year&order_by=numeric') ?></span>
</nav>

http://ww.wp.xz.cn/plugins/ambrosite-nextprevious-post-link-plus/