Once I log out the changes are gone

Where can we see the website in question?

https://www.pervasivedisplays.com/

What other plugins and theme do you use on the site?

Plugins: Advanced Custom fields PRO, Anti-Spam, Classic Editor, Contact Form 7 (and Dynamic Text Extension, Submissions, ReCapture v2), YouTube Embedded, Enable Media Replace, NitroPack, Post SMTP, Safe SVG, Search & Filter Pro, TablePress, Theme Editor, UpdraftPlus, Yoast SEO.

Theme: our own CSS

Thanks.

Try switching to the default theme, deactivating all plugins excluding Contact Form 7, Classic Editor and Yoast SEO, and clearing all cache data.

Why you are advised to deactivate plugins and switch to the default theme.

@takayukister Thanks for your hint.
After checking with each plugin, there is NitroPack needs to purge the cache. Everything works as normal now.
#closed