The reason for that is in located in the method fields_view_get_address in /addons/base/res/res_partner.py.

The partner views are dynamically changed based on the country of the current company. The tag <div class="address_format"> is replaced by one of the following values (depending on the address_format of the country):

    layouts = {
        '%(city)s %(state_code)s\n%(zip)s': """
            <div class="address_format">
                <field name="city" placeholder="City" style="width: 50%%"/>
                <field name="state_id" class="oe_no_button" placeholder="State" style="width: 47%%" options='{"no_open": true}'/>
                <br/>
                <field name="zip" placeholder="ZIP"/>
            </div>
        """,
        '%(zip)s %(city)s': """
            <div class="address_format">
                <field name="zip" placeholder="ZIP" style="width: 40%%"/>
                <field name="city" placeholder="City" style="width: 57%%"/>
                <br/>
                <field name="state_id" class="oe_no_button" placeholder="State" options='{"no_open": true}'/>
            </div>
        """,
        '%(city)s\n%(state_name)s\n%(zip)s': """
            <div class="address_format">
                <field name="city" placeholder="City"/>
                <field name="state_id" class="oe_no_button" placeholder="State" options='{"no_open": true}'/>
                <field name="zip" placeholder="ZIP"/>
            </div>
        """
    }

I suggest to disable this "feature" by applying one of the following methods:

1. Override the function fields_view_get_address:

   class res_partner(osv.osv):
      _inherit ='res.partner'
      def fields_view_get_address(self, cr, uid, arch, context={}):
           return arch
   res_partner()
   

2. Update the address_format in the country of your company:

   Goto menu "Sales/Configuration/Address Book/Localisation/Countries" and open the country of your company. Then you have to assure that the addess_format does not match one of the above formats. E.g. for Austria you can replace

   %(street)s
   %(street2)s
   %(city)s %(state_code)s %(zip)s
   %(country_name)s
   

with

    %(street)s
    %(street2)s
    %(city)s XXX %(state_code)s %(zip)s
    %(country_name)s