Hi John,
I’ve disabled all other plugins and switched to 2017 theme, but it’s still not working – odd!
Here is a screen grab of the web inspector, just to show that i have the relevant field key and default value.
add_filter('acf/load_field/key=field_58086a9b13be5', 'kiteboat_set_tax_default');
function kiteboat_set_tax_default( $field ) {
$field['default_value'] = 304; // that's "private"
return $field;
}