in odoo14, when set placehold of many2one field in form, but the placehold do not display.
<odoo>
<data>
<record id="trade_employee" model="ir.ui.view">
<field name="name">trade.employee.form</field>
<field name="model">hr.employee</field>
<field name="inherit_id" ref="hr.view_employee_form"/>
<field name="arch" type="xml">
<field name="job_title" position='replace'>
<field name="job_id" placeholder="Job position"/>
</field>
</field>
</record>
</data>
</odoo>
i have tried it in my Odoo 14 instance and works well.
Make sure you have pulled the latest build of odoo14 and your field is not replaced somewhere ( You may want to add a string to your field in XML to make sure that it is exactly the one displayed in the form view, ex string="JOB" ).
Hope it helps.