Ans: Invoice Line Ids fields disappears or cannot store At once

<xpath expr="//field[@name='invoice_line_ids']/tree/field[@name='product_id']" position="after" >
    <field name="ksc_re_invoice1"/>
xpath>
 <xpath expr="//field[@name='line_ids']/tree/field[@name='product_id']" position="after" >
    <field name="ksc_re_invoice1" />
xpath>

from above code you can easily store values of invoice line fields or itwould not disappears.

Thank you