NginX redirect Frontend to different port and path

For some very specific reasons I have to access two different routes on my frontend and the only variable in how they are accessed I have control over is the port. So what I would like to do is depending on the port, either access the frontent normally, or access it under a different path:

myUrl:80 -> myUrl:80
myUrl:8081 -> myUrl:80/someRoute

I have tried to implement this with a proxy_pass as follows:

# Standard frontend server
server {
    listen 80 default_server;
    server_name _;

    root /usr/share/nginx/html;
    index index.html;
    error_page 500 502 503 504 /50x.html;

    location / {
        try_files $uri $uri/ /index.html
        add_header Cache-Control "no-cache";
    }

    location /static {
        expires 1y;
        add_header Cache-Control "public";
    }
}

# Proxy $url:8081 -> url:80/somePath/
server {
    listen 8081 default_server;
    server_name _;

    location / {
        proxy_set_header Host $host;
        proxy_set_header X-Original-Host $http_host;
        proxy_set_header X-Original-Scheme $scheme;
        proxy_set_header X-Forwarded-For $remote_addr;

        proxy_pass http://127.0.0.1:80/somePath/;
    }

}

This then throws an error in the frontend console:

The script has an unsupported MIME type ('text/html').

In case it's relevant, I'm using Flutter Web.