I’m in a similar situation but unresolved by the suggestions above. A sample of the empty menus below. This takes place no matter which menu item I select. I also re-installed the older version of the plugin and had the same results.
https://snipboard.io/UIQMuj.jpg
The site is live, not staging
Cloudflare cache was purged
Autoptimize on the front end. Purged cache to no effect
Wordpress 5.5
Themify Ultra theme