From your first post, the argument in $new_startup
is correct.
Have you tried the following the check on what the $post_id is?
function sl_acf_pre_new_startup( $post_id ) {
write_log('POST ID: '.$post_id);
}
add_filter( 'acf/pre_save_post' , 'sl_acf_pre_new_startup', 10, 2 );