【関連記事設定－関連性＝カテゴリ】
【表示中の・投稿ID=1234・カテゴリ=9】
【カスタム投稿 post_type=hogehoge/taxonomy=hoge_tag,hoge_cat】

■やってみたこと

【cocoon-child-master/functions.php】に追記
function hoge_search_filter($query) {
if (is_singular( 'hogehoge' ) ) {
$query->set( 'post_type', array( 'post', 'hogehoge' ) );
}
}
add_action( 'pre_get_posts','hoge_search_filter' );

■結果ダンプ
【cocoon-master/tmp/related-list.php】
１.$args = get_related_wp_query_args()　内容ダンプ
(term_taxonomy/taxonomy='hoge_categories')
-----------
  'post__not_in' => 
    array
      0 => int 1234
  'posts_per_page' => int 5
  'orderby' => string 'rand'
  'no_found_rows' => boolean true
  'date_query' => 
    array
      0 => 
        array
          'after' => string '2020-01-01 0:0:0'
          'inclusive' => boolean true
  'category__in' => 
    array
      0 => int 9
-----------
→これは合ってるぽい
※関連をカテゴリーで指定、その他指定内容も加味されている

２.  $query = new WP_Query( $args ); 後に$query->query_varsダンプ
-----------
  'post__not_in' => 
    array
      0 => int 1234
  'posts_per_page' => int 5
  'orderby' => string 'rand' 
  'no_found_rows' => boolean true
  'date_query' => 
    array 
      0 => 
        array
          'after' => string '2020-01-01 0:0:0'
          'inclusive' => boolean true
  'category__in' => 
    array 
      0 => int 9
  'error' => string '' 
  'm' => string '' 
  'p' => int 0
  'post_parent' => string '' 
  'subpost' => string '' 
  'subpost_id' => string '' 
  'attachment' => string '' 
  'attachment_id' => int 0
  'name' => string '' 
  'pagename' => string '' 
  'page_id' => int 0
  'second' => string '' 
  'minute' => string '' 
  'hour' => string '' 
  'day' => int 0
  'monthnum' => int 0
  'year' => int 0
  'w' => int 0
  'category_name' => string '' 
  'tag' => string '' 
  'cat' => string '' 
  'tag_id' => string '' 
  'author' => string '' 
  'author_name' => string '' 
  'feed' => string '' 
  'tb' => string '' 
  'paged' => int 0
  'meta_key' => string '' 
  'meta_value' => string '' 
  'preview' => string '' 
  's' => string '' 
  'sentence' => string '' 
  'title' => string '' 
  'fields' => string '' 
  'menu_order' => string '' 
  'embed' => string '' 
  'category__not_in' => 
    array
      empty
  'category__and' => 
    array
      empty
  'post__in' => 
    array
      empty
  'post_name__in' => 
    array
      empty
  'tag__in' => 
    array
      empty
  'tag__not_in' => 
    array
      empty
  'tag__and' => 
    array
      empty
  'tag_slug__in' => 
    array
      empty
  'tag_slug__and' => 
    array
      empty
  'post_parent__in' => 
    array
      empty
  'post_parent__not_in' => 
    array
      empty
  'author__in' => 
    array
      empty
  'author__not_in' => 
    array
      empty
  'post_type' => 
    array
      0 => string 'post'
      1 => string 'hogehoge'
  'ignore_sticky_posts' => boolean false
  'suppress_filters' => boolean false
  'cache_results' => boolean true
  'update_post_term_cache' => boolean true
  'lazy_load_term_meta' => boolean true
  'update_post_meta_cache' => boolean true
  'nopaging' => boolean false
  'comments_per_page' => string '50'
  'order' => string '' 
-----------
→これも合ってるような気がする

３.  $query->requestダンプ
-----------
SELECT   posts.ID FROM posts  WHERE 1=1  AND ( 
  posts.post_date >= '2020-01-01 00:00:00'
) AND posts.ID NOT IN (1234) AND ( 
  0 = 1
) AND posts.post_type IN ('post', 'hogehoge') AND (posts.post_status = 'publish' OR posts.post_author = 1 AND posts.post_status = 'private') GROUP BY posts.ID ORDER BY RAND() LIMIT 0, 5
-----------

→なにか色々違うぞ
・0 = 1：[category__in]が上手く翻訳できなかった？

何卒宜しくお願い致します。