In the ODF plugin, some classes use node.isNodeType() on nodes which can be frozen nodes. In that case, the value returned by isNodeType() is "nt:frozenNode" and the real nodeType is in the jcr:frozenPrimaryType property. Same goes with mixins types, UUID, etc.
Example in the Container class :

Node node = (Node) it.next();
if (node.isNodeType(SchoolPath.NODE_TYPE))
{
    children.add(new SchoolPath(node));
}
...
else if (node.isNodeType(TeachingUnitElement.NODE_TYPE))
{
    children.add(new TeachingUnitElement(node));
}

If the node iterator is retrieved from a JCR query which returns frozen nodes, the nodes aren't recognized as SchoolPath or any other type, and not processed.