@anthony - One more issue that made me waste a lot of time and resort to workarounds (that later I had to undo): when a record is saved during Preview, it sets _owner to a certain GUID, but that differs from wixUsers.currentUser.id. So I can save a record but not retrieve it if my dataSet filter is set to “Owner is logged in user”.
Since Preview runs as admin this is expected but I can see how this is confusing. Will share this feedback.