Triage update:

Under 2.4.8, the planner generates a single indexed solution when a hint is provided.
Under 2.5.4, a collection scan is generated in addition to the indexed solution. It's possible for the collection scan to be ranked higher than the indexed solution, resulting in the hinted solution being ignored.

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mongo --shell test.js
> use dummy
switched to db dummy
> doit()
BasicCursor

Expected to see "BtreeCursor ace.val_1 multi" output.