Thanks for reporting the issue, I hope to resolve this in 2-3 days.
This document seems to contain two strange (damaged) drop down form fields that MS Word does not really show as drop down fields, but it shows their field codes if you use Alt+F9 to toggle display of field codes. The drop down fields are after text" staff, or mail it to: , " on page 6. You can workaround if you delete these fields. I’ve added resiliency logic to ignore such damaged fields, will release in Aspose.Word 3.3.2 soon.