@Scott_Chapman @barmac The problem stems from the fact that the example diagram declares default and prefixed usage for the same XML namespace:
xmlns:tns="http://www.jboss.org/drools"
xmlns="http://www.jboss.org/drools"
While this is not illegal usage it seems to confuse our importer regardless. As the diagram at hand does not use the default namespace it is safe to remove it. Afterwards the diagram imports (and serializes) without issues.
The files load just fine (a bunch of warnings, but it works).
The “just fine” warnings are reporting exactly what the importer is confused about.