We ran into an issue today, and did find the answer, but I'm opening this as a discussion because I know there must be a best practice, or at least a better way.
We have multiple projects running at any given time, but they are all on the same sprint and release cycles. Our PM/BA team doesn't want to have to create a Sprint 10 (for example) in a dozen separate projects, so what they started doing is creating a sprint in one project, then using that same sprint in all of the others. This is great, except that they aren't managing the projects from a single unified board; they're each creating their own boards in their own projects. That results in the issue we had today, where someone tried to close a sprint and was asked by Jira if they wanted to move the unresolved issues into the next sprint. The PM was confused, because they didn't see any unresolved issues in their board; it turned out the issues were from another project that they weren't displaying on their board.
One thought I had, which is a major kludge, is to have a separate project called Sprints. That project wouldn't have any issues of its own. All sprints would be created in that project, and there would be a single backlog and active sprint board that would combine issues from all projects. Each PM/BA could maintain their own boards in their own projects to monitor progress and move issues towards completion, but the actual sprints would be managed by a single PM and closed by that same PM. That way, we'd avoid the issue that occurred today.
What are all of your thoughts on this? Is there a better way? (We do have Portfolio, but we're not great about estimating yet, and it's kind of overwhelming. At this point, it also feels like swatting a fly with a Cadillac - a bit overkill, based on our needs.)