In general Missing namespace information is the hint that you did not provide the camunda namespace information to the model:
xmlns:camunda=“http://camunda.org/schema/1.0/bpmn”
Refer to this test case to learn how to manually supply it.
You mentioned the diagram already contains the information. If that is the case, we may look at a bug. Please open a ticket in bpmn-moddle, including a minimal test case and we may be able to have a look.