Default Data Deduplication Optimisation job with "StopWhenSystemBusy" does not work correctly

I have configured Data Deduplication on Windows Server 2012 R2 and enabled on a volume.

Data Deduplication creates a default Optimisation task called "BackgroundOptimization".

I have placed files on my volume, and started the job manually.  It succeeds but does nothing.  InPolicyFiles still shows zero.

If I edit this job in Task Scheduler and remove the /backoff switch (StopWhenSystemBusy), then start the job manually again, it succeeds, and also incrememnts the InPolicyFiles.

At the same time, the CPU and RAM on the server is fairly idle, with no users on the system.

Is there a problem with the StopWhenSystemBusy?  Should we not use this parameter, and simply run the task out of business hours instead?  Has anyone seen this before?

This happens on two Windows Server 2012 R2 servers, so is reproducable here.

David