why php7.4 uses fastcgi_pass unix:/var/lib/php7.2-fpm/web2.sock ?

Hi

My sever default PHP ver is 7.2 and I installed PHP 7.3, 7.4.
I have site assigned php 7.4 and from php info also shows 7.4
why the site vhost file setting using fastcgi_pass unix:/var/lib/php7.2-fpm/web2.sock; not 7.4?

This site from time to time no response, and error 502 bad gateway (other sites on the same server no problem), server cpu loading very low.

from httpd log I saw
2020/12/01 15:57:01 [error] 1186#1186: *1424 recv() failed (104: Connection reset by peer) while reading response header from upstream, client: 13.13.92.19, server: mysite.com, request: "GET / HTTP/2.0", upstream: "fastcgi://unix:/var/lib/php7.2-fpm/web2.sock:", host: ....

location @php {
try_files $uri =404;
include /etc/nginx/fastcgi_params;
fastcgi_pass unix:/var/lib/php7.2-fpm/web2.sock;
fastcgi_index index.php;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
fastcgi_intercept_errors on;
}