If you create a subtask the due date of the top level task is automatically adjusted to include the subtask range. For example, if my task has a due date of 9/2 and I set a subtask with a due date of 11/1, the due date of the task changes from 9/2 to 11/1.
This is indeed not a thing (yet?), subtasks and parent tasks are never related in terms of completion and dates. In my case, I would not want to have that feature