Your question has made me realise that the issue is probably not with the ACF part but the Custom Post Types taxonomy I was querying.
get_field is correctly pulling the field ID. However, my $args was not querying for posts filtered by taxonomy. I’ve changed the code to:
<?php $relatedCat = get_field('service_related_industry');
$args = array (
'posts_per_page' => 3,
'post_type' => 'case_study',
'tag_id' => '"'.$relatedCat.'"',
'orderby' => 'DESC',
It’s still not working properly, but at least now it isn’t pulling all the posts (it’s pulling exactly the 3 latest, while ignoring the ‘tag_id’ filter).
