These queries generate FLWOR expressions with an order by, but no for loop, eg:

let $i := (1, 3, 2)
order by $i
return $i

I think they allow a response, as they don't expect the orderspec to be evaluated as there is no for-loop to do the order by over. But I think it is valid to still evaluate the orderspec, and hence throw an err:XPTY0004 as $i doesn't have a single value.