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_config.py
import uuid
from odoo import api, fields, models, _
from odoo.exceptions import UserError
class AccountCashboxLine(models.Model):
    _inherit = 'account.cashbox.line'
    default_pos_id = fields.Many2one('pos.config', string='This cashbox line is used by default when opening or closing a balance for this point of sale')
class AccountBankStmtCashWizard(models.Model):
    _inherit = 'account.bank.statement.cashbox'
    
    @api.model
    def default_get(self, fields):
        vals = super(AccountBankStmtCashWizard, self).default_get(fields)
        config_id = self.env.context.get('default_pos_id')
        if config_id:
            lines = self.env['account.cashbox.line'].search([('default_pos_id', '=', config_id)])
            if self.env.context.get('balance', False) == 'start':
                vals['cashbox_lines_ids'] = [[0, 0, {'coin_value': line.coin_value, 'number': line.number, 'subtotal': line.subtotal}] for line in lines]
            else:
                vals['cashbox_lines_ids'] = [[0, 0, {'coin_value': line.coin_value, 'number': 0, 'subtotal': 0.0}] for line in lines]
        return vals
class PosConfig(models.Model):
    _name = 'pos.config'
    def _default_sale_journal(self):
        return self.env['account.journal'].search([('type', '=', 'sale'), ('company_id', '=', self.env.user.company_id.id)], limit=1)
    def _default_pricelist(self):
        return self.env['product.pricelist'].search([], limit=1)
    def _get_default_location(self):