Impacted versions:
* 11.0

Steps to reproduce:
* Create company A
* Create company B
* Admin user with both companies but currently in company B
* Create Demo user only with company B
* Create fiscal position A to company A
* Create fiscal position B to company B
* Create vendor with fiscal position A with company A and the same vendor with fiscal position B in company B
* Create product and add the vendor created before
* Set like **Create a draft purchase order**
* Logging with Demo user
* Create sale order with one sale order line with route **make to order**
* Validate sale order

Current behavior:
The purchase order created by sale order has the fiscal position **company A**

Expected behavior:
The purchase order must have fiscal position **company B**
addons/purchase/models/purchase.py
for order in self:
            if order.picking_ids and all([x.state == 'done' for x in order.picking_ids]):
                order.is_shipped = True
    READONLY_STATES = {
        'purchase': [('readonly', True)],
        'done': [('readonly', True)],
        'cancel': [('readonly', True)],
    }
    name = fields.Char('Order Reference', required=True, index=True, copy=False, default='New')
    origin = fields.Char('Source Document', copy=False,\
        help="Reference of the document that generated this purchase order "
             "request (e.g. a sale order or an internal procurement request)")
    partner_ref = fields.Char('Vendor Reference', copy=False,\
        help="Reference of the sales order or bid sent by the vendor. "
             "It's used to do the matching when you receive the "
             "products as this reference is usually written on the "
             "delivery order sent by your vendor.")
    date_order = fields.Datetime('Order Date', required=True, states=READONLY_STATES, index=True, copy=False, default=fields.Datetime.now,\
        help="Depicts the date where the Quotation should be validated and converted into a purchase order.")
    date_approve = fields.Date('Approval Date', readonly=1, index=True, copy=False)
    partner_id = fields.Many2one('res.partner', string='Vendor', required=True, states=READONLY_STATES, change_default=True, track_visibility='always')
    dest_address_id = fields.Many2one('res.partner', string='Drop Ship Address', states=READONLY_STATES,\
        help="Put an address if you want to deliver directly from the vendor to the customer. "\
             "Otherwise, keep empty to deliver to your own company.")