Currently it's only possible to manually assign, either in Portfolio or Jira, a story-level issue to a single sprint while stages could be done in different sprints.

When letting Portfolio calculate the issues this work "within Portfolio" but as the manual assignment is not possible, this can't be reflected in Jira / Boards.

It is not viable to use sub-tasks to split the work as sub-tasks can't be assigned to a different sprint that its parent.

Workaround is to either create multiple stories for each stage and use issue links to represent the relation, or use a level of hierarchy above for the main task and to cut it down per stage at the story level.