Phabricator's custom form preview doesn't actually hide the hidden fields, instead it shows them at 50% opacity. It looks cluttered and doesn't provide a very useful "preview" at all. One line of css will fix the issue.
Upstream Task in Wikimedia Phabricator:
https://phabricator.wikimedia.org/T209743
Upstream change that could be imported:
https://phabricator.wikimedia.org/rPHABbba62cf5243538af9e37cc1211a01d247294f9f7