ERR_SSL_PROTOCOL_ERROR quando o frontend acessa o backend

O frontend abre em https, acessa o backend em my_domain/api/start, mas a resposta não é retornada e há um erro no console:

GET https://my_domain.com:8082/api/start net::ERR_SSL_PROTOCOL_ERROR

Aqui está meu nginx.conf:

server {
    listen 80;
    server_name my_domain.com www.my_domain.com;
    return 301 https://$server_name$request_uri;
}

server {
    listen 443 ssl;
    server_name my_domain.com www.my_domain.com;

    root /usr/share/nginx/html;
    index index.html;

    ssl_certificate /etc/nginx/ssl/certificate.crt;
    ssl_certificate_key /etc/nginx/ssl/private.key;

    ssl_protocols TLSv1.2 TLSv1.3;
    ssl_ciphers ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:HIGH:!aNULL:!MD5;

    # API Backend proxy
    location /api/ {
        proxy_pass http://backend:8082;
        proxy_http_version 1.1;
        proxy_set_header Upgrade $http_upgrade;
        proxy_set_header Connection 'upgrade';
        proxy_set_header Host $host;
        proxy_cache_bypass $http_upgrade;
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
    }

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

    error_page 404 /index.html;
}

Minha composição:

services:
  frontend:
    build:
     context: ./frontend
     args:
      NGINX_CONF: ${NGINX_CONF}
#    network_mode: host
#    depends_on:
#      - backend
    ports:
      - '80:80'
      - "443:443"
    volumes:
      - ./frontend/ssl:/etc/nginx/ssl
    environment:
      - ENVIRONMENT=${ENVIRONMENT}
  backend:
    build:
     context: ./backend
     args:
      CONFIG: ${CONFIG}
    ports:
      - '8082:8082'
      - '8443:8443'
    volumes:
      - ./frontend/ssl:/etc/nginx/ssl
    environment:
      - ENVIRONMENT=${ENVIRONMENT}

networks:
  app-network:
    driver: bridge

curl por http://my_domain:8082/api/start responde normalmente, sem erros. Tentei muitas variantes e nada muda.