Actually the merge field is there. What you see is the result mode of the field.
You could change the field display mode, in RadRichTextBox and MS Word, by clicking with the mouse right button on the field and pressing the "Toggle Field Codes" option in the context menu.
In case you need to preserve the fields in DisplayName mode you could do it by using the DocxExportSettings.FieldResultMode. Here is example on how to do it:
DocxFormatProvider formatProvider =
formatProvider.ExportSettings.FieldResultMode = FieldDisplayMode.DisplayName;
Here is a link to useful help article about the Import/Export settings
. And here is another article you may find useful as well about Mail Merge
I hope this information answers your questions.
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