OK think this is because the get_posts()
function defaults to the post
post type.
To test this we can try adding the post_type argument. In the PHP you will find two instances of:
$args = array(
'posts_per_page' => 1,
'include' => $prevPost->ID,
);
Add the additional argument with your post_name so its like this:
$args = array(
'posts_per_page' => 1,
'include' => $prevPost->ID,
'post_type' => 'YOUR-POST-TYPE-NAME-HERE'
);
I still think the category argument will cause an issue so leaving them as stated here should be the left as is:
https://generatepress.com/forums/topic/custom-navigation-on-cpt/page/2/#post-826332