In case anyone here wants to make their own utility for this purpose, mine relies mainly on two things: a custom version of the acf_update_bidirectional_values() function that sets $current_values to an empty array (which I call my_acf_update_bidirectional_values(), and then the following custom function.
This function accepts two inputs. $posts is an array of the posts you want to iterate over (the ones containing the correct values), retrieved via get_posts(); $fields is a simple array of the field keys or names that hold the bidirectional relationships.
function my_acf_bidi_sync($posts=[], $fields=[]) {
foreach ((array)$posts as $post) {
foreach ((array)$fields as $fkey) {
$field = get_field_object($fkey, $post->ID);
if (!empty($field['value'])) {
$mapped_ids = array_map(function($v) { return $v->ID; }, $field['value']);
my_acf_update_bidirectional_values($mapped_ids, $post->ID, $field);
}
}
}
}
