Well, the pre_get_posts filter you have is probably interfering with other queries. What you need to do is to limit it more.
Instead of just checking to see if it’s the admin do something like
if (is_admin() || !$query->is_main_query()) {
// limit to only the main query
return;
}
You may also need to do further limiting, for example limiting it to a specific post type
if (!isset($query->query_vars['post_type']) ||
$query->query_vars['post_type'] != 'post') {
return;
}
the code examples given here are meant for reference and simple examples and may not be everything you need.
