We haven’t any reports of the issue you described. Can you update your Ninja Forms plugins to their latest versions and let me know if you continue to have the same issues?
Can you temporarily disable your plugins, minus the Ninja Forms plugin and its add-ons, and set your theme to a WordPress default theme such as Twenty-Sixteen, to see if you are having the same issue? If you do not have a development environment for your site (this is best practice and highly recommended), the Health Check plugin (https://wordpress.org/plugins/health-check/) can disable plugins and theme for a single user session instead of all users on your site.
Can you also contact the WP Fastest Cache support (https://wordpress.org/support/plugin/wp-fastest-cache) to see if they know the cause of this issue.
