1.) Right-click the sample XML file and select "Properties"
2.) Click the "Advanced" button in the "General" tab. A new configuration window opens.
Could you confirm if the box "Encrypt contents to secure data" is checked?
Most cases, XML files are pre-encrypted & I think, XML Encryption, also known as XML-Enc, is always on.
My best guess & hopefully the closest one, is that DLP does not scan encrypted XML files, especially the ones encrypted & not available in free text for content inspection.