First of all, please make sure that the form fields in the mentioned PDF file contains caption. Secondly, please try to use the latest version of the merged Aspose.Pdf for .NET. Kindly note that the version 6.0.0 and above are part of the merged Aspose.Pdf. Lastly, if that doesn’t help either then please share the input PDF file with us, so we could test the issue at our end and guide you accordingly.
We’re sorry for the inconvenience.