The attached test shows that a many-to-one association referencing an unique property (instead of the Id) results in a select n+1 problem. Although the first statement contains the correct join, all associated entities are fetched one by one after the join select. (Entities with the same value in the unique column are even fetched more than once.)
The interesting point is that this bug only occurs if the referenced entities are already in the session cache. If they are not, no additional select statements are created.