Subtask link is removed when reordering tasks on the Agile Board

I have a Task and its Subtask both on the same column of the Agile Board.  The Task card initially appears above the Subtask card, as shown below:

If I then choose to reorder these cards by dragging the SubTask card to above the Task card, the subtask link between the two issues is removed, i.e. it is no longer the subtask of the other task.

Interestingly, if I had dragged the other way around by instead dragging the Task card to below the Subtask card, then the subtask link remains in place.

Is this correct behaviour, or am I not using subtasks correctly?

(I'm using YouTrack 7.0 standalone.)

Thanks,

Peter.