Just wondering if its a bug?
...good question. :) Any opinion on this?
Yes I have an opinion; Yes it should copy userData. IMO.
As of version 2.5.3, TiXmlElement::CopyTo( TiXmlElement* target ) is inconsistent. It copies the TiXmlElement::userData, through its base class TiXmlNode::CopyTo( target ). However, it does not copy the TiXmlAttribute::userData.