301 Redirect from Subdomain to Main Domain After Node.js Proxy Setup (HestiaCP + NGINX + Apache)

Hello, I’m experiencing a persistent redirect issue on my server running HestiaCP.

Setup Overview:

HestiaCP is installed on a Hetzner VPS.

Apache is listening on ports 8081 and 8543.

NGINX is used as the reverse proxy.

Let’s Encrypt is used for SSL.

WordPress is installed under a subdomain: old.example.com.

The Problem:
After configuring the proxy for example.com, I started getting an unexpected redirect:

When accessing https://old.example.com/wp-login.php, it redirects with 301 to https://example.com/wp-login.php.

What I’ve already checked:

siteurl and home values in the WordPress database for old.example.com are correctly set to https://old.example.com.

.htaccess is clean, no rewrite or redirect rules.

I renamed the plugins directory to disable all WordPress plugins.

No redirect rules are found in /etc/nginx/conf.d, /etc/nginx/sites-enabled, or any NGINX include paths for old.example.com.

Verified only example.com is proxied to Node.js (grep -r “proxy_pass http://127.0.0.1:3000”).

SSL broke across domains temporarily after the proxy config. I had to manually restart NGINX and Apache to partially recover.

Question:
Why does visiting old.example.com cause a 301 redirect to example.com when no such rule exists?
Is this behavior due to a conflict between the Node.js proxy config and how HestiaCP handles NGINX virtual hosts?

Any ideas or guidance would be very appreciated.

Thanks.