Ok, in that case, if it’s feasible, I would strongly suggest trying to move your workflow to the task level instead of the subtask level (leveraging either project sections or projects or custom fields to group the related tasks instead of heaving them under a task. Subtasks can be really tricky to deal with as they lack a lot of the flexibility of tasks (case in point: the issue you’ve identified here). I have a similar workflow and we moved all our subtasks to top-level tasks in sections that corresponded to the old parent task.
If that’s just absolutely not possible for your workflow, you can convert subtasks to tasks (which you can then filter/move), but it’s a little wonky. I believe the only ways are: 1) open the task pane and manually drag/drop the subtasks into a section of the project, or 2) use the API (e.g., in a script action) to set the parent task to null and add the “subtask” back to the project). Obviously neither of those are ideal.
For a more permanent solution, you can vote for this thread: