xsd:Any: Missing namespace declaration in output for element with with non-prefixed namespace xmlns=""

The attached input gpx file (Schema at http://www.topografix.com/GPX/1/1/gpx.xsd) will be output without a namespace declaration on the TrackExtension element in an unmarshal-marshal round trip.
The same input will be properly serialized if a prefixed namespace declaration
xmlns:gpxx="http://www.garmin.com/xmlschemas/GpxExtensions/v3"
is used instead of the default namespace declaration
xmlns="http://www.garmin.com/xmlschemas/GpxExtensions/v3"