AWS portal - Spot fleet worker instances not shutdown after idle interval

I have a (fairly) new Deadline 10.1.17.4 installation with AWS Portal. I am testing the creation and use of a spot fleet of 10 x “Deadline 10 Worker” instances, setting the “Auto Shutdown instances” to 5 minutes (default). The worker instances are not being shutdown after the workers idle status exceeds that 5 minute limit. The Workers panel is displaying the status of the 10 AWS Workers (currently idle for 38 mins). The AWS Portal link and Asset Server status is Online, and the AWS Fleet is healthy.

I can manually stop the Spot Fleet and retry with the same result.

Am I misunderstanding the purpose of the “Auto Shutdown Instances” option of Spot Fleet Configuration? My assumption was that the instances would be terminated after the idle time on any given Worker exceeds that 5 minute mark, and recreated when a job is scheduled that requires the fleet resource.

Thanks for any assistance. - Martin