Parent - Subtask relationship is not showing on team-managed projects when issues are imported from CSV

Issue Summary

When you import a parent issue like a story with its subtasks using the external system import CSV the subtasks are not showing inside the parent issue, however, when you do an advance search the subtasks show a parent relation.

Steps to Reproduce

1. Export or create a CSV file with a parent issue and its subtasks
2. Use the external system import to import the CSV file into a Next-gen project
   https://share.getcloudapp.com/NQu1e0wm

Expected Results

The subtasks should be displayed inside the parent ticket or in the board when grouped by subtask

Actual Results

No subtasks are displayed in the parent ticket just in the advanced search

Workaround

Recreate the subtasks or first import all the issues into a Classic project and then move them to a Next-gen
Note: Trying to move them to reassign the parent is not working.