Hi, does anyone know why this extra filed does not appear, 

I have other <record> with extra field in the same xml file and that works fine.

        <record model="ir.ui.view" id="view_pos_pos_form_inherit">

            <field name="name">pos.order.inherit</field>

            <field name="model">pos.order</field>

            <field name="inherit_id" ref="point_of_sale.view_pos_pos_form"/>

            <field name="arch" type="xml">

                <form string="Point of Sale Orders">

                    <sheet>

                        <xpath expr="//field[@name='session_id']" position="after">

                            <group col="4" colspan="4">

                                <field name="x_exp_date"/>

                            </group>

                         </xpath>

                    </sheet>

                </form>

            </field>

        </record>