Thanks for the reply!
Here is the filter function.
function sl_acf_pre_new_startup( $post_id ) {
if( $post_id != 'new' ) :
write_log('not new');
return $post_id;
else:
write_log($post_id);
return $post_id;
endif;
}
add_filter( 'acf/pre_save_post' , 'sl_acf_pre_new_startup', 10, 2 );
Using the above, it only ever returns “not new” even though it’s creating (successfully) a new post.