According to your documentation what I describe here has been implemented by design.
When a sprint has ended, stories that haven't been completed go back to the top of the backlog. If you have any sprints planned though the top represents the next planned sprint!!
According to Scrum basics a story should go back to the backlog once it hasn't been resolved. Regarding agile boards the story should be positioned right after the backlog marker and not the first sprint.

I know that an improvement already exists GHS-6421 but in my opinion (and the customer I represent) zhis should be treated as a bug.