Including text fields is a part of MS Word that allows us to reuse common components. If this feature is used, it is not able to be updated via GemBox.Document. I am not familiar with a workaround for this issue.
According to this link below you claim to be releasing more updatable field types. Is there a timeline for IncludeText fields?
https://www.gemboxsoftware.com/document/docs/GemBox.Document.Field.html#GemBox_Document_Field_Update
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