Hi Felix -
Yes, completed subtasks are do not show up in the Archived section until all their parent tasks are completed to.
As you observed, completed subtasks can be seen in Task Overview while their parent tasks are still open. Details here:
Clicking the task overview on the top-level parent task will show you all the subtasks, whether open or completed (the magnifying glass icon only appears on tasks that have subtasks).
You can adjust the "Checkmark Action" to also control whether subtasks are archived or just crossed off so they are easier to keep track of:
-Cameron