Wikimedia Bugzilla is closed!

Actually it's PHP mangling the names, as the same behavior is seen in a four-line test page that just echos back $_GET and $_POST.

 <?php
 header( 'Content-Type: text/plain' );
 var_dump($_GET);
 var_dump($_POST);

Besides space, this also affects period and open-bracket (and ASCII NUL truncates the key), at least in PHP 5.5.1 on my local system. With respect to periods and spaces, this behavior is mentioned at http://www.php.net/manual/en/language.variables.external.php.

Changed back to getting the raw POST and parsing it ourselves, so this is no longer an issue.