I think there shouldn't be much of a difference between a task and a subtask, that a subtask is still just a task. Like there should be a way to view a task, similar to how we view a project, where the task is displayed in the center pane, and selecting a subtask will display the task details in the right pane.
We have deep hierarchy of tasks and subtasks, and sometimes we convert a task into a project just for that view purpose, which then makes the sidebar flooded with so many projects.
Actually, going back to your issue, I think it's not only in Asana board the behavior happens. I created a search view to get all incomplete tasks coming from 4 different projects filtering all subtasks. The task under project A, which is a subtask of a task in project B was filtered out.