Query with windowing function and ORDER BY isn't using covering index

Index: 

CREATE INDEX `wf3` ON `bucket-1`((`item`.`manufacturer_id`),(`item`.`brand`),`sales_price`)

Query 1:  Windowing function + ORDER BY.
Latency is 6 sec

{{SELECT item.brand, }}
{{SUM(sales_price) AS item_revenue, }}
COUNT(1) OVER () AS items_per_brand
FROM `bucket-1`
WHERE item.manufacturer_id = 12
{{GROUP BY item.brand }}
ORDER BY item_revenue;

Query2: No windowing function
Latency is 300ms

{{SELECT item.brand, }}
SUM(sales_price) AS item_revenue
FROM `bucket-1`
WHERE item.manufacturer_id = 12
{{GROUP BY item.brand }}
ORDER BY item_revenue;

Query3: No GROUP BY
Latency is 300ms

SELECT item.brand,
SUM(sales_price) AS item_revenue,
COUNT(1) OVER () AS items_per_brand
FROM `bucket-1`
WHERE item.manufacturer_id = 12
GROUP BY item.brand

Query Plans attached.

Logs of the full test run with Query 1:

http://perf.jenkins.couchbase.com/job/arke/398/

[https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-398/172.23.97.18.zip
] https://s3-us-west-2.amazonaws.com/perf-artifacts/jenkins-arke-398/172.23.97.19.zip