I create with Activiti Modeler a Text Annotation.
Then I save the model and export it to a file. Result : the text annotation is missing from the bpmn20.xml file.
Activiti Modeler generates a process in JSON format (TextAnnotations are still present).
Activiti creates a BpmModel. During the parsing of the Json element, TextAnnotations are not considered.
This bug concerns the architecture of the model. I think you need to represent TextAnnotation in the JsonConverterModel (maybe modify the BpmnModel ....).