Hi,

Odoo's default behavior is to abort the operations when a backorder MO is created. Odoo considers that the operations from the original MO cannot be completed as they were and hence cancels them when you establish a backorder MO. This is because the system considers that the remaining operations are no longer valid. After all, the quantity of the initial MO has changed.
Create a custom module or add your code to an existing module.
Inherit the mrp.production model to modify the behavior of creating backorder MOs.
Override the action_backorder method in your custom module.

from odoo import models

class MrpProduction(models.Model):
    _inherit = 'mrp.production'

    def action_backorder(self):
        backorder_moves_vals = []
        for move in self.move_finished_ids.filtered(lambda x: x.state not in ('done', 'cancel')):
            if move.product_uom_qty <= 0.0:
                continue
            backorder_moves_vals.append(move._prepare_move_copy_values(move.product_uom_qty))
        if backorder_moves_vals:
            backorder_production = self.copy({
                'move_raw_ids': [],
                'move_finished_ids': backorder_moves_vals,
                'backorder_id': self.id,
                'date_planned_start': False,
                'date_planned_finished': False,
                'state': 'draft',
                'workorder_ids': [],
                'move_dest_ids': [],
            })
            backorder_production._onchange_move_raw()
            backorder_production.action_assign()
        return True

Regards

I was also experiencing the same problem.  

Two options:

1. Without customization, you can delete the cancelled work orders in the back ordered MO then add work orders again.  This provides flexibility as to what specific work orders to start with.
2. With customization, you can add a button, to recopy the work orders from BOM (automating #1), delete work orders that are not needed.