files inside wp-includes are automatically modifying leading to website crash

There seems to be some kind of issue going on with WordPress website (my version 5.8.3) where the files inside the wp-includes folder are getting modified automatically. This is causing the website to crash (‘There has been a critical error on your website’). I doubt if this is a 0 day.

Some of the files that I found getting affected are:

pluggable-deprecated.php
 class-wp-meta-query.php
 class.wp-styles.php
 rewrite.php
 class-walker-page.php

And these are the files which are throwing error in the server logs.

FastCGI sent in stderr: "PHP message: PHP Fatal error:  Uncaught Error: Function name must be a string in /var/www/html/public_html/wp-includes/pluggable-deprecated.php:1
Stack trace:
#0 /var/www/html/public_html/wp-settings.php(424): require()
#1 /var/www/html/public_html/wp-config.php(92): require_once('/var/www/html/p...')
#2 /var/www/html/public_html/wp-load.php(50): require_once('/var/www/html/p...')
#3 /var/www/html/public_html/wp-admin/admin-ajax.php(22): require_once('/var/www/html/p...')
#4 {main}
  thrown in /var/www/html/public_html/wp-includes/pluggable-deprecated.php on line 1" while reading response header from upstream, client: redacted, server: *.redacted.com, request: "POST /wp-admin/admin-ajax.php HTTP/1.1", upstream: "fastcgi://unix:/var/run/php/php7.4-fpm.sock:", host: "redacted.com", referrer: "https://redacted.com/wp-admin/"