Java JDOM2 XML add Element to existing child

Question

I am trying to insert a new child at a certain position in an existing XML document. To do this, I have first parsed the existing XML document and now want to edit and save it.

In the imports i use

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.jdom2.Element;
import org.jdom2.Document;
import org.jdom2.Namespace;
import org.jdom2.input.DOMBuilder;
import org.jdom2.output.DOMOutputter;

This is how I parse the files and convert them to jdom2 format

DocumentBuilder builder = null;
org.jdom2.input.DOMBuilder jdomBuilder = new DOMBuilder();
Document tmpDoc = jdomBuilder.build( builder.parse(new FileInputStream(tmpFilePath)));

This is how i try to edit and save the files

    Document tmpDoc = archivedFiles.get(id);
    Element eleWithTracking = (Element) new Element("TRACKING_ID").addContent(idAndTracking.get(id));
    
    Element root = (Element) tmpDoc.getRootElement();
    Element header = (Element) root.getChild("ORDER_HEADER", Namespace.getNamespace("http://www.opentrans.org/XMLSchema/2.1"));

    header.addContent(eleWithTracking);
    tmpDoc.setContent(root);

    try {
        //Create the XML
        XMLOutputter outter=new XMLOutputter();
        outter.setFormat(Format.getPrettyFormat());
        outter.output(tmpDoc, new FileWriter(new File(id + "_withTracking.xml")));            
    } catch (IOException ex) {
        Logger.getLogger(Reverse_nes.class.getName()).log(Level.SEVERE, null, ex);
    }

However, I cannot find the child I just inserted (eleWithTracking) anywhere in the output file.

Furthermore, the output file is not formatted in the same way as the source file. I don't have any nodes there that I can expand, but only pure text.

Unfortunately I don't know Java that well yet and i don't get any error messages.