Starting with Jira 7, sprint and ranking data is migrated with the rest of the project data. This works nicely with the feature in Project Configurator that migrates Scrum or Kanban boards, as part of the project configuration, providing a complete solution for the migration of Jira Software projects.
However, there is a problem when there are sprints that include issues for more than one project. In this situation, these sprints will be duplicated (one sprint will be created for each project involved) and the ranks will only keep the relative order among issues belonging to the same project. This is a result of the underlying data transport technology importing one project at a time. Project Configurator just calls this process once for every project to be imported.