I could not see anything simpler than the_field() or get_field(). You should not bypass that point without cleaning your script.
You should test (in the top of the script or the loop) if the related post in the single post page has the ‘parent_series’ set:
<?php
die(get_field('parent_series');
?>
Or maybe your problem is simply related with a misunderstanding of the WordPress loop which could easily leads to strange behaviour like the one your are talking about.
