Currently, if a status is mapped to "done" in JIRA but doesn't have a resolution post function or the workflow associated to the project or issue type removes the resolution on transition, the issue will NOT show in Portfolio, unless the child issue is in progress or to do. It would be nice if Portfolio ignored the resolution date when brining in issues.
Expecting all of our customers to use canned workflows and resolutions on done statuses is very "perfect world JIRA".
We have documentation on this but it is one line:

Any issues that have been marked as done in Jira Software, that don't have a resolution set, will not be included in the plan.

It is confusing for end users and adds technical debt for JIRA admins to open a ticket with Atlassian to run down what the issue might be.