Impacted versions: 
9.0x

Steps to reproduce:
1. Go to Accounting, create customer invoice and validate. Check the Invoice-Number (for example: INV/2016/0010)
2.  Go to Point of Sale and select New Session
3. Add any product or just close & confirm the session
4. Close the POS Session (Validate Closing & Post Entries)
5. Create new customer invoice and validate it. Check the Invoice-Number. 

Current behavior:
Gap between the first and the last invoice because of the closed POS-Session. 

Expected behavior:
No Gap between the two invoices like in Version 8. 

I've tested this behaviour on a 8 and a 9 runbot and my own system. There is no gap on 8 but on 9. I think this is not a feature but a bug. I can't find the missing sequence-numbers in any Journals or in the invoice-list. 

The only way to avoid this, is to select another Sales Journal in the POS-Configuration. But then you can't create an invoice from the POS. 

Is it possible to get the "lost" invoice-numbers somewhere? We are not allowed to have gaps in the invoice-numbering here in Germany. 

addons/point_of_sale/models/pos_order.py
"""
        Prepare the dict of values to create the new invoice for a pos order.
        """
        return {
            'name': self.name,
            'origin': self.name,
            'account_id': self.partner_id.property_account_receivable_id.id,
            'journal_id': self.sale_journal.id,
            'type': 'out_invoice',
            'reference': self.name,
            'partner_id': self.partner_id.id,
            'comment': self.note or '',
            
            'currency_id': self.pricelist_id.currency_id.id,
            'user_id': self.env.uid,
        }
    def _action_create_invoice_line(self, line=False, invoice_id=False):
        InvoiceLine = self.env['account.invoice.line']
        inv_name = line.product_id.name_get()[0][1]
        inv_line = {
            'invoice_id': invoice_id,
            'product_id': line.product_id.id,
            'quantity': line.qty,
            'account_analytic_id': self._prepare_analytic_account(line),
            'name': inv_name,
        }
        
        invoice_line = InvoiceLine.sudo().new(inv_line)
        invoice_line._onchange_product_id()