I found this topic from 2019 which was solved. But was trying to make this work with sub fields. i.e. – What if the Events fields were repeater fields that could be populated multiple Places fields (Map/Address fields, etc.). I gave it a shot below, but I’m missing something.
https://support.advancedcustomfields.com/forums/topic/clone-field-value-from-relationship-post/#post-135706
add_action('acf/save_post', 'save_event_update_address', 20);
function save_event_update_address($post_id) {
if (get_post_type($post_id) != 'events') {
// not and event post, bail
return;
}
// get location post ID
// you said it's a relationship field, a relationship field will turn an array of posts
// I am setting the 3rd parameter because we only need the IDs
$related_places = get_sub_field('relationship_field_name');
// also the
if ($related_places) {
$place_id = $related_places[0];
$address = get_field('address_field_on_place_name', $place_id);
update_sub_field('address_field_on_event_repeater_name, $address, $post_id);
}
}