When invoking the Jenkins rest api for Freestyle jobs, the response began returning a BOM appended to the beginning:
<?xml version="1.0" encoding="UTF-8"?>

This is causing malformed XML.
I know this script was working in my last known jenkins ver : 1.627

Also, interestingly enough, this seems to only be a problem with freestyle jobs, retrieving the config.xml for my Maven jobs return normal XML data.

Powershell :

$url = "http://jenkins:8080/job/JobName/config.xml"
$cnfg = Invoke-RestMethod -Headers @{Authorization=("Basic {0}" -f $base64AuthInfo)} -Uri $url -method GET