I have the same issue – ACF functions do not seem to work in update_post, regardless of priority.
The solution for me, because I needed to use post_updated to compare old and new post titles and slugs, was to update the value in $_POST[‘acf’]. E.g:
$_POST['acf']['field_5f19d4ad03a30'][] = ['field_5f19d4b503a31' => $old_title]
In this case, I am saving the old title to a repeater. For this to work you must use field keys, not field names.
