ZUGFeRDExporterFromA3 -> setXML removes existing Metadata fields like title

[thomass4t]

We have a PDF file which contains XMP metadata like title, creator etc.
When attaching ZUGFeRD as XML, the XMP metadata gets lost.

Sample code:

        try (ZUGFeRDExporterFromA3 zea3 = new ZUGFeRDExporterFromA3().set)
        {
            zea3.load(pdfFile.getAbsolutePath());
            zea3.setXML(xml);

Going through the code, the Metadata gets lost during this code in ZUGFeRDExporterFromA3

	protected void prepareDocument() throws IOException {

		PDDocumentCatalog cat = doc.getDocumentCatalog();
		metadata = new PDMetadata(doc);
		cat.setMetadata(metadata);

		removeCidSet(doc);
		xmp = getXmpMetadata();

The line "cat.setMetadata" overwrites the existing metadata with an empty MetaData Object.

Unfortunately, I can't tell why these two lines exist and what the purpose is.

With ZUGFeRDExporterFromA3 I can only set producer and creator, but all existing MetaData like title are lost.
It would be great if somebody with experience could take a look at this part.
Maybe it would be enough to use the existing metadata (if present) instead of always setting a complete new metadata object.

[thomass4t]

Short addendum:
I modified prepareDocuments() and added blank metadata only if it's empty:

    protected void prepareDocument() throws IOException
    {
        PDDocumentCatalog cat = doc.getDocumentCatalog();
        metadata = cat.getMetadata();
        if (metadata == null || metadata.getLength() == 0)
        {
            metadata = new PDMetadata(doc);
            cat.setMetadata(metadata);
        }
        removeCidSet(doc);
     ...

This seems to work and preserves existing metadata.