Nested subtasks were returned in the order their relations were created, so
they ignored the view's own order - reordering a subtask by dragging it, or
sorting the view, left the nesting untouched.
Positions only exist per view, and not every caller reads tasks through one:
the task detail page, CalDAV and exports pass no view, and a subtask in another
project has no row for this view either. Those keep relation id order, which
stays stable, and sort after the ones that do have a position.