The idea is that rank is managed within a sprint (or backlog) itself, not across sprints.
In other words: Issues are only re-ranked (upon being moved) when there are other issues to compare them with in the same sprint (or backlog) where they've been moved to. If other issues have already been added to the sprint, the ranking of the issue will be relative to the ranking of the other issues in the sprint. When you move an issue from the Scrum backlog to a sprint, and it's the first issue added to the sprint, the issue will retain its rank. You can only rank issues if ranking has been enabled (see Enabling ranking), and if you have the 'Schedule Issue' and 'Edit Issue' permissions for the issue you want to move.
