I have the same problem and cennot solve it. I tried to inspect the testDoc-file but it is marked private.
Can I please get access to TestDocs.zip to see how exactly the fields are embedded into a Word doc?
Please see TestDocs.zip (17.7 KB) and try running the following code:
Document doc = new Document("D:\\temp\\input.docx");
doc.getMailMerge().execute(new String[]{"show"}, new String[]{"true"});
doc.save("D:\\temp\\awjava-20.1.docx");
Hope, this helps.