We have extended the module res.partner. We have added a field after the email, and added a method on_change on this new field. In the method on_change it create an instance of a TransientModel class, and when it make the method self.env['test.classì].create({}), odoo set to null the fields related to retail Accounting.
How to help me understand the reason of this behavior?

code: 

# -*- coding: utf-8 -*-
from openerp import models, fields, api
class res_partner(models.Model):
    _inherit = 'res.partner'
    test = fields.Char(string='Test')
    @api.onchange('test')
    @api.one
    def onchange_test(self):
        p = self.env['test.class'].create({})
        p.a = "test"
        return

class test_class(models.TransientModel):
    _name = "test.class"
    a = fields.Char()
    b = fields.Char()