Planned date for backorder
Currently, the planned date is set using the Function field shipping_date/delivery_date. But those function fields return existing date once a move is linked to the line. So if the order has to create a backorder, it will reuse the same date as the original move as planned date.
I do not think it is right. The nature of backorder is that they are created when something wrong happens (missing stock, supplier delay etc.).
For me, we can not set a planned date for backorder because we are exactly in the unknown.
I propose to not set a planned date for supplier backorder like that it will not be used for supply etc. But of course the user can set a planned date manually.
I propose to set a planned date to today for customer backorder like that it generates ASAP the request for the product.