I've created a new plan, filling it with a JIRA project. There are 4 releases and most of the issues are already planed for a release, other are floating and I'm ok Portfolio makes suggestion for them.
My issue is that the schedule proposed by Portfolio is mixing all releases along the year (I'm not sure from which criteria, maybe priority?). So some issues of the release 1 starts after release 2 or 3, etc.
Thanks in advance,