While Asana continues to work on this feature (or ignore us completely) here’s a hack that might come in handy -
The auto-hiding of completed subtasks does not work in the List view but it does work in the Board view. You can toggle between seeing all tasks and incomplete tasks - the filter also gets applied to subtasks automatically.
This makes me think this is a bug in the list view that has needed fixing for approx. 3 yrs now.