If you set the attribute's namespace here (i.e. http://www.tei-c.org/ns/1.0), Oxygen shows up an error message:
It's seems, that Oxygen has a problem with the "http://" in the namespace. Without this "http://", the attribut is inserted correctly with the namespace-prefix.The attribute does not have a valid xml name: ":rend"
A valid attribute name must match the standard specification. http://www.w3.org/TR/xml/#NT-Name
If you don't set the namespace, all works fine, but the attribute is in the namespace of the belonging element. That's intended in our case, so it isn't a problem in the moment.