With this SPARQL query:
We get the following algebra:
The issue is that the order is on rating, however rating is not bound until later in the algebra, just prior to the projection. This yields un-ordered results.
Checking this with ARQ, we get the algebra:
There the bind for the expression is prior to the ordering, which I believe is the correct interpretation.
I've attached a sample input file that can be used for execution which, if you execute the query with that data, you should get data that looks something like
You'll notice the ratings are not correctly ordered.