Impacted versions:
11.0

I don't know if it is an issue, but I think is strange.

`product_qty` is in unit of `product` : https://github.com/odoo/odoo/blob/11.0/addons/stock/models/stock_move.py#L240
`reserved_availability` is in unit of `stock.move` : https://github.com/odoo/odoo/blob/11.0/addons/stock/models/stock_move.py#L295

`move.product_qty - move.reserved_availability` seems to have no sens, no ?

https://github.com/odoo/odoo/blob/11.0/addons/stock/models/stock_move.py#L915
https://github.com/odoo/odoo/blob/11.0/addons/stock/models/stock_move.py#L917
https://github.com/odoo/odoo/blob/11.0/addons/stock/models/stock_move.py#L924

thank for your answer

@jco-odoo 
@alnslang 


addons/stock/tests/test_move.py
done linked stock move and ensure the scrap quantity is also
        updated.
        """
        self.env['stock.quant']._update_available_quantity(self.product1, self.stock_location, 10)
        partner = self.env['res.partner'].create({'name': 'Kimberley'})
        picking = self.env['stock.picking'].create({
            'name': 'A single picking with one move to scrap',
            'location_id': self.stock_location.id,
            'location_dest_id': self.customer_location.id,
            'partner_id': partner.id,
            'picking_type_id': self.env.ref('stock.picking_type_out').id,
        })
        move1 = self.env['stock.move'].create({
            'name': 'A move to confirm and scrap its product',
            'location_id': self.stock_location.id,
            'location_dest_id': self.customer_location.id,
            'product_id': self.product1.id,
            'product_uom': self.uom_unit.id,
            'product_uom_qty': 1.0,
            'picking_id': picking.id,
        })
        move1._action_confirm()
        self.assertEqual(move1.state, 'confirmed')
        scrap = self.env['stock.scrap'].create({
            'product_id': self.product1.id,
            'product_uom_id': self.product1.uom_id.id,
            'scrap_qty': 5,
            'picking_id': picking.id,
        })
        scrap.action_validate()