1. 

Make sure that the mf_ids field is correctly defined as a Many2many field in both the product.product model and the sale.order.line model.

class SaleOrderLine(models.Model):

    _inherit = 'sale.order.line'

    mf_ids = fields.Many2many(

        'product.mf.pgm',

        string="Manufacturing Programs",

    )

   class ProductProduct(models.Model):

    _inherit = 'product.product'

    mf_ids = fields.Many2many(

        'product.mf.pgm',

        string="Manufacturing Programs",

    )

Try this:

@api.onchange("product_id")

def _fetch_mf_ids(self):

    for rec in self:

        if rec.product_id:

            mf_ids = rec.product_id.mf_ids

            rec.mf_ids = [(6, 0, mf_ids.ids)]

Hope it helps