Hi, thanks for your reply.
It now seems to post without running the function and therefore not unzipping the file. If I change the following:
//$post_id = wp_insert_post( $post );
add_filter('acf/save_post' , 'pre_save_post', 1 );
It still posts but doesn’t unzip the file and doesn’t display ‘TESTING’.
I have debug set true but have no errors.