I noticed that my css and js files were being served without the charset in the content type…

content-type: text/css;

It should be (source)…

content-type: text/css; charset=utf-8

To resolve, login to Plesk and navigate to your domain, and click Apache & nginx Settings, scroll down to Additional nginx directives and add…

charset UTF-8;
charset_types text/plain text/css text/xml application/json application/manifest+json application/javascript application/rss+xml;

as you can see, you can just list the file types as you would for gzip_types.