one of the values you are using in the multiple update is the pg_id column of the WADApage_content recordset.
the WADApage_content recordset uses the pg URL Variable for filtering. When the page first loads, that variable is present, but your forms action does not send that variable when you post the form. when the form is posted, that recordset is recreated, and since the pg URL Variable no longer exists, the recordset is empty.
use the following code for the form action so that it will retain any url variables passed in:
action="<?php echo (htmlentities($_SERVER["PHP_SELF"], ENT_QUOTES)); ?><?php echo (isset($_SERVER["QUERY_STRING"]) && $_SERVER["QUERY_STRING"] != ""?"?".htmlentities($_SERVER["QUERY_STRING"], ENT_QUOTES):""); ?>"