Issue hierarchy can only be configured at global level in current version of Portfolio and this impacts all the plans being setup in that instance of Portfolio. While this may work for organizations having project of similar nature and scale, it's not very tenable for organizations with projects of differing nature (Hybrid Agile, Scrum, SAFe, etc) and scale (project, program, portfolio, enterprise). To support a varied setup, it will be helpful to configure issue hierarchy at the plan level (rather than at global level).