php 8.1 fast-cgi always showing the same web site

Which website you see is not bound to the PHP version on the config side. This behavior is quite strange indeed. What happens if you change the PHP version of one of these site to e.g. PHP 8.0 or 8.2, does it start working then and you see the correct site? Also, do the vhost files of these sites get stored with a .err file ending in the web server sites-available folder?

When you see the wrong website, this normally happens due to wrong IP / domain / SSL (port) combinations as described here: https://forum.howtoforge.com/threads/please-read-before-posting.58408/ in chapter 2. The only possibility I see in regard to getting such behavior in PHP is that the sites might be specifically programmed to use a shared PHP cache or so, which is made to re-use the content of other sites and, therefore, delivers the content of another site if not implemented properly in the site's code.