Testing in an OOB instance, the same behavior was seen. If moving un-completed Stories to the Product Backlog, the sprint Percent Complete did not update to 100%. The only time the Percent Complete went to 100% was when moving the stories to Closed Complete, this would update the Percent Complete to 100% on the sprint and when moving them to the Product Backlog, the 100% complete stayed.

It looks like the Stories must first be closed complete and that will make the sprint move to 100% complete.