@margotseedsandstones-org there is nothing in your code that should be causing the error you are seeing. get_sub_field() does not cause the do_shortcode() function to be called nor is there anything in getting or formatting the value of a relationship field that would cause do_shortcodes() to be called.
However, the relationship field could be returning an array rather than a single post and calling one of the sequestration WP functions with the wrong value might be causing the error. You should check what is returned here:
$postobject = get_sub_field('related_pages');
echo '<pre>'; print_r($postobject); echo '</pre>';
