I just had a look through the code in ACF and here is the issue
FILE: includes/api/api-helpers.php
function acf_get_post_types()
LINE: ~642
// Bail early if is builtin (WP) private post type
// i.e. nav_menu_item, revision, customize_changeset, etc.
if ( $object->_builtin && ! $object->public ) {
continue;
}
There is a filter
return apply_filters( 'acf/get_post_types', $post_types, $args );
and you could use this filter to add ‘post’ back in.