WSO2 Payload Factory not working as expected - xpath

Question

I am having troubles configuring the Payload Factory to make it work as expected.
I am trying to process the response of a backend service, which is:
<Documents xmlns="http://ws.wso2.org/dataservice">
<Document>
<Data>
{ "_id" : { "$oid" : "5bbce6ec9e0aae7e5c3a150a"} , "Plan" : "XXXX"}
</Data>
</Document>
<Document>
<Data>
{ "_id" : { "$oid" : "5bbce7279e0aae7e5c3a150b"} , "Plan" : "YYYY"}
</Data>
</Document>
</Documents>
I need to extract the json in each Data tag and construct a JSON that looks something like:
{
Data:
{
_id: {...},
Plan: ...
}
}
Just for testing purposes, I was trying to use the Payload Factory Mediator to get all Data tags using XPath. This is the outSequence of my API (again, the response that comes from the backend is just like the one above):
<outSequence>
<payloadFactory media-type="xml">
<format>
<newTestData>$1</newTestData>
</format>
<args>
<arg evaluator="xml" expression="//Data" />
</args>
</payloadFactory>
<log level="full" />
</outSequence>
The problem is that the log shows that the newTestData tag is empty after the Payload Factory process the response message.
The XPath was tested in an XPath Online Tester and it is correct so: what am I doing wrong?