This bug is still valid, the bug appear only if the selected field is the last of its own stack !!!
:
        <record id="view_partner_category_form_partner" model="ir.ui.view">
            <field name="name">res.partner.category.form.partner</field>
            <field name="model">res.partner.category</field>
            <field name="inherit_id" ref="base.view_partner_category_form"/>
            <field name="arch" type="xml">
                <xpath expr="/form/field[@name='parent_id']" position="after">
                    <newline />
                    <separator colspan="4" string="Partners"/>
                    <field colspan="4" name="partner_ids" nolabel="1"/>
                </xpath>
            </field>
        </record>

it leads to this from a server answer to a view ask (in the DEBUG_RPC log):

<form string="Partner Category">
    <field name="name" select="1"/>
    <field name="active" select="1"/>
    <field name="parent_id"/>
    <field colspan="4" name="partner_ids" nolabel="1"/>
    <separator colspan="4" string="Partners"/>
    <newline/>
</form>

Indeed in code (server/bin/osv/orm.py @ line 1261):
                   else:
                       sib = node.getnext()
                        for child in node2:
                            if pos == 'inside':
                                node.append(child)
                            elif pos == 'after':
                                if sib is None:
                                    node.addnext(child)                                    
                                else:
                                    sib.addprevious(child)
                            elif pos == 'before':
                                node.addprevious(child)
                            else:
                                raise AttributeError(_('Unknown position in inherited view %s !') % pos)

Our nodes are like this (DOM):
+ name
+ active
+ parent_id (the "node" var in code)

The actual code will do this:
1. <newline />
   * sib = None (the node is the last of its stack)
   * add after "node"
   * result DOM:
      + name
      + active
      + parent_id
      + newline
2. <separator colspan="4" string="Partners"/>
   * sib = "newline node"
   * add BEFORE "newline node"
   * result DOM:
      + name
      + active
      + parent_id
      + separator
      + newline
3. <field colspan="4" name="partner_ids" nolabel="1"/>
   * sib = "separator node"
   * add BEFORE "separator node"
   * result DOM:
      + name
      + active
      + parent_id
      + partner_ids
      + separator
      + newline

The patch to correct this is the following one (server/bin/osv/orm.py @ line 1261) :
                    else:
                        for child in node2:
                            if pos == 'inside':
                                node.append(child)
                            elif pos == 'after':
                                node.addnext(child)
                                node = child
                            elif pos == 'before':
                                node.addprevious(child)
                            else:
                                raise AttributeError(_('Unknown position in inherited view %s !') % pos)

This is simpler and work as expected in all cases !