Solved! Go to Solution.
I believe the issue here with this use-case is Marketo considers boolean fields that are unchecked as actually having a FALSE/NO value. Which is why prefill should be turned off for all hidden boolean fields.
Yes, it should. The problem being that it is so easy to ignore it, driving users to forget about it. It is especially painful when you discover after a few months that hundreds of people who have filled out the "unsubscribe" from are still "unsubscribe = false" in the DB...