Hello mvpevents,
Sorry to hear you are having issues with your Ninja Forms. When testing, I am not experiencing any issues. I would like to try and see if I can duplicate the issue.
Which theme are you using? Does this occur with all ninja forms? Are you viewing an Active or Staged theme? When you say preview, are you viewing from the page editor or are you clicking in view from the dashboard?
Kindest Regards,
Scott M