Nginx converts POST Request to GET request while proxy_pass

Question

I am having 3 Server: A, B, C. Details are:

1. Server A: NGINX Server, URL: https://test1.example.com
2. Server B: NGINX Server hosting NodeJS Web Application, URL: http://test2.example.com
3. Server C: Apache2 Server hosting Django Web Application, URL: http://test3.example.com

Server A(NGINX Server) is front facing public facing server acting as load balancer using proxy_pass. Server B(NGINX Server) is having a form and submitting it through POST request to Server C(UWSGI Django Server using Apache2). The request is from Server B with url: https://test1.example.com/register to Server C but Server A is converting it to GET request.

So I am getting 2 request log one with POST and other with GET with same path "/register"

The setup is depicted in figure (Diagram is at the end of the post).

The configuration is as follows:

1. Server A:

   server {
   listen 443 ssl http2;
      listen [::]:443 ssl http2;
      server_name test1.example.com;
   ssl_certificate /etc/ssl/test1/test1.example.com.crt;
   ssl_certificate_key /etc/ssl/test1/test1.example.com.key;
   
   
   Proxy/Loadbalancer configuration`
   Testing the request`
       location / {
         proxy_pass http://test2.example.com;
       }
   
       location /register/{
               proxy_pass http://test3.example.com;
       }
   
   }
   

2. Server B:

   server {
   listen 80 default_server;
   
   listen [::]:80 default_server;
   
   root /var/www/html;
   
   # Add index.php to the list if you are using PHP
   index index.html index.htm;
   
   server_name test2.example.com;
   
   location / {
           #try_files $uri $uri/ =404;
            try_files $uri $uri/ /index.html;
   }
   
   access_log /var/log/nginx/access.log;
   error_log  /var/log/nginx/error.log;
   
   

   }

3. Server C:

   <VirtualHost *:80>
   ServerName  test3.example.com
   ServerAdmin admin@example.com
   DocumentRoot /var/www/test
   Alias /static/  /var/www/test/webapp/static/
   Alias /media/  /var/www/test/media/
   WSGIScriptAlias /   /var/www/test/webapp/wsgi.py
   WSGIDaemonProcess   cdac.in  python-path=/var/www/test 
   
       python-home=/var/www/test/venv  processes=5   threads=8
   WSGIProcessGroup cdac.in
   <Directory /var/www/test/webapp/>
       Options -Indexes
       Order deny,allow
       Allow from all
       Require all granted
   </Directory>
   LogLevel info
   PROJECT_NAME is used to separate the log files of this application
   ErrorLog    /var/log/apache2/error.log
   CustomLog   /var/log/apache2/access.log combined
   

Where am I going wrong.