Apache 2.4.x Reverse Proxy subdirectory and ports issue?

I have 3 different applications deployed on the same server. Each on different ports

1. One java API application running on port 8080. directory (/home/api/)
2. Second nextjs web app running on port 3000. directory (/home/web)
3. Third vuejs admin panel application on default port 80 deployed inside subdirectory (/var/www/html/admin)

This is an apache config file test.conf

    ServerName www.test.com

    ServerAdmin webmaster@localhost
    DocumentRoot /var/www/html

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

    # Setup reverse proxy for all server
    ProxyPreserveHost On

    ProxyPass /admin http://localhost/admin
    ProxyPassReverse /admin http://localhost/admin

    ProxyPass /api http://localhost:8080/api
    ProxyPassReverse /api http://localhost/api

    ProxyPass / http://localhost:3000/
    ProxyPassReverse / http://localhost/

API is working fine at http://test.com/api URL.

The website is also working fine at http://test.com URL.

The problem occurs when I access http://test.com/admin URL.

It shows the following error in browser:-

Your browser sent a request that this server could not understand. The size of a request header field exceeds the server limit.

with status code 400 Bad request and If I remove the admin panel from reverse proxy and create another vhost file with a simple configuration like below:-

This is another apache vhost config file test-admin.conf

    ServerName www.test.com

    ServerAdmin webmaster@localhost
    DocumentRoot /var/www/html/

    #LogLevel info ssl:warn

    ErrorLog ${APACHE_LOG_DIR}/error.log
    CustomLog ${APACHE_LOG_DIR}/access.log combined

     <Directory /var/www/html/admin>
            Options Indexes FollowSymLinks
            AllowOverride All
            Require all granted
            Order allow,deny
            Allow from all
    </Directory>

then other URL stops working showing 404 Not Found error.

Note:- www.test.com is just to represent the actual domain name or IP.