Hi,
I had a look at the transition_post_status action on the codec, looks like it’s “not intended for use by plugin and theme developers”.
if you $value is a static value I think what you want is pretty straight forward with ACF.
// run AFTER ACF saves the $_POST['fields'] data
// here we run the acf/save_post AFTER the data are saved and we update the field to your $value
add_action('acf/save_post', 'my_acf_save_post', 20);
function my_acf_save_post( $post_id ){
$value = "12345";
update_field('field_534dc7420db07', $new_tax, $post_id);
}
That way you ensure that the $value stays always the same, it can be on newly published posts or on post that goes from pending to published, as soon as the post is saves the field_534dc7420db07 gets your value after it was saved.
you should leave the acf.php as it was and shouldn’t comment anything on that part it will end up breaking things.
then if you want to make more complicated stuff you can check if the post is published or pending or any other stuff using it’s id that is stored in $post_id
