HiddenItem not included in form POST (Mobile SmartGWT)

I am using Mobile Smart GWT (2013-12-04 build) and when I use HiddenItems within a form they are never included in the POST sent to the server. TextItem and other FormItem elements are sent in the message but no matter what I do the HiddenItem items are not sent to the server. I looked at the raw message and they are missing.

I am submitting the form with a POST and using Multipart (as part of a file upload).

Thank you