We were not able to reproduce the problem you described - we performed a test by setting a data source to an enumerable containing an ExpandoObjects and inserted some merge fields randomly in large document:
dynamic expando =
i = 0; i < 10; i++)
document.MailMergeDataSource.ItemsSource = Enumerable.Repeat(expando, 1);
Do you have any additional code that modifies the document in your project? If so, you can try to isolate the problem in sample project and send it to us attached to a support ticket, so that we can investigate the issue further.
All the best,
the Telerik team
Explore the entire Telerik portfolio by downloading the Ultimate Collection trial package. Get it now >>