Good point. And I can’t see a way to find incomplete Subtasks of complete parents with Asana Advanced Search.
Also, I can’t tell from a Task list with Subtasks listed or from the Subtask’s pane if the parent Task is complete or not. An indication from both locations would be helpful.
Unless someone is more clever and can find a way to do it. I would suggest moving this to Product Feedback for voting.
A workaround for not completing the parent Task of incomplete Subtasks is to make the parent dependent on one or more of the Subtasks. As discussed in this topic.
And if you haven’t already, please vote for these Subtask improvements.