you for using Syncfusion products.
Update fields behavior (CTRL+A, F9) of MS Word is already implemented in DocIO by calling the UpdateDocumentFields() method. MailMerge.Execute() will performs replacements of every merge field in the document with data from the data source. If you call the UpdateDocumentFields() method before executing the mail merge the fields are updated using existing values. After executing the mailmerge, DocIO uses the populated values to update the fields.
In your given nested if field scenario we have found the issue in updating field result and we have forwarded this to our developer for further analysis. We have created a new incident #126574 for further follow ups. Kindly follow the incident #126574 to get resolved the mentioned issue.
Please let us know if you have any questions.