Nginx is decoding "%2F" on proxy pass

Question

I am trying to set up a Nginx reverse proxy for accessing an API located at port 3000, inside a Docker container.

The proxy works fine for many kinds of urls, including those with escaped characters such as %20 (white space), %3C (<), %3E (>), etc. However, when it comes to the character %2F (/), Nginx decodes it before forwarding the request to the corresponding server.

For instance, the URL:

https://example.com/api/August%2F2021/games/desc/

becomes:

https://example.com/api/August/2021/games/desc/,

thus returning a 400 bad request.

This is my nginx.conf:

user nginx;
worker_processes 1;
error_log /var/log/nginx/error.log warn;
pid /var/run/nginx.pid;

events {
  worker_connections 1024;
}
http {
  include /etc/nginx/mime.types;
  default_type application/octet-stream;
  log_format main '$remote_addr - $remote_user [$time_local] "$request" '
                  '$status $body_bytes_sent "$http_referer" '
                  '"$http_user_agent" "$http_x_forwarded_for"';
  access_log /var/log/nginx/access.log  main;
  sendfile on;
  keepalive_timeout  65;
  server {
    listen [::]:443 ssl ipv6only=on; # managed by Certbot
    listen 443 ssl; # managed by Certbot
    ssl_certificate /etc/letsencrypt/live/haxmania.com.br/fullchain.pem; # managed by Certbot
    ssl_certificate_key /etc/letsencrypt/live/haxmania.com.br/privkey.pem; # managed by Certbot
    include /etc/letsencrypt/options-ssl-nginx.conf; # managed by Certbot
    ssl_dhparam /etc/letsencrypt/ssl-dhparams.pem; # managed by Certbot
    server_name example.com; # managed by Certbot

    location / {
      root /app;
      index index.html;
      try_files $uri $uri/ /index.html;
    }

    location /api/ {
      proxy_pass http://backend:3000/;
    }

    error_page 500 502 503 504 /50x.html;

    location = /50x.html {
      root /usr/share/nginx/html;
    }
  }
  server {
    if ($host = example.com) {
      return 301 https://$host$request_uri;
    } # managed by Certbot

    listen 80;
    listen [::]:80;
    server_name example.com;
    return 404; # managed by Certbot
  }
}

My question is almost identical to this one, however none of the solutions worked for me. I tried changing from:

proxy_pass http://backend:3000/; to proxy_pass http://backend:3000$request_uri;

but it didn't not work.

I believe the solution might be writing a rewrite directive, but I am struggling on how exactly this should be done... Any tips would be appreciated.