I too have this problem. I use to be able to use the Radio group and get the form to validate UNTIL I STARTED TO USE UNIVERSAL EMAIL with Form Builder. Sorry to shout but I need to make that point really loud and clear.
To be honest I'm really angry about having to constantly post stuff here instead of being able to open a Support Ticket myself. It has taken me days to get a form to work by constantly having to post here and wait for a reply. I told a customer over a week ago that creating a form would only take a day. Now I feel like I am really incompetent as I cannot get any solutions.
Thanks for listening WA Support but please can I get a Support Ticket started myself????? When I was told that I got free support for signing up to the extensions library I didn't know it was going to take days to get something to work.