Hidden form-tag not passing to form submission

I have two fields that I would like to have hidden and when I use the [hidden text] tag on the field it hides the field but the problem is the data is not showing up in flamingo and I’m not able to access in my custom before_send_mail event.
If I take the hidden tag of the field then all works as it I would expect it to.
Any ideas what might be causing the hidden tag to work this way?

Thanks
Kirk