nginx reverse proxy with docker-compose passes to wrong container

I have a network of 3 containers set up in a docker-compose.yml. 2 containers are web applications and the third is a reverse proxy. All 3 containers run nginx. This is the nginx.conf for the reverse proxy:

events {}
http {
    server {
        listen       8090;
        server_name  localhost;
        location / {
            proxy_pass http://myappui:8080;
        }
        location /admin/ {
            proxy_pass http://myappadmin:8081/;
#  this works  proxy_pass http://microsoft.com/;
        }
        }
    }

The problem is that the proxy_pass for the location /admin/ somehow routes calls to the first container as http://myappui:8080/admin rather than passing to the second container as http://myappadmin:8081/. If, for location /admin/, I replace myappadmin:8081 with microsoft.com, it works and the calls are correctly routed to microsoft.com.

If I call the second container myappadmin with the automatically assigned host port, it opens the myappadmin application correcly.

This is my docker-compose:

version: '3.4'

services:
  myappui:
    build:
      dockerfile: ./Docker/Dockerfile.ui
    tty: true
    container_name: myappui
    ports:
    - "8080"
    networks:
    - webnet

  myappadmin:
    build:
      dockerfile: ./Docker/Dockerfile.admin
    tty: true
    container_name: myappadmin
    ports:
    - "8081"
    networks:
    - webnet    

  nginx: 
    image: nginx:latest
    tty: true
    container_name: myappreverseproxy
    ports:
    - "8090:8090"
    volumes:
      - ./nginx.conf:/etc/nginx/nginx.conf
    networks:
    - webnet

networks:
  webnet  :
    driver: bridge