Portfolio's JQL function "portfolioChildIssuesOf" does not fetch all transitive children of issues in classic JSD projects
Have a classic JSD project with at least one issue with an issue type mapped to Portfolio's Epic and one on Story level. Those should have a parent-child relation.
Make a Portfolio plan with said project and verify the parent-child relation is properly shown in the Scope table.
Go to the issue search and try "portfolioChildIssusOf($issueKeyOfParent)"
The child is found by the issue search.
Nothing is found by the issue search.
Currently there is no known workaround for this behavior. A workaround will be added here when available