haproxy doesn't seem to see all the backends/replicas

I'm trying to make haproxy proxy requests to a number of replicas:

docker-compose.yml:

services:
  haproxy:
    image: haproxy:2.3-alpine
    volumes:
      - ./haproxy.cfg:/usr/local/etc/haproxy/haproxy.cfg
    ports:
      - 8888:80
  app:
    build: .
    command: perl app.pl
    init: true
    deploy:
      replicas: 10

haproxy.cfg:

global
    maxconn  1024
listen in
    bind :80
    server-template srv 10 app:8080 check
    timeout connect 5000ms
    timeout client 50000ms
    timeout server 50000ms

But it seems to see only some of the replicas:

$ docker compose up
...
app-3      | MyWebServer: You can connect to your server at http://localhost:8080/
haproxy-1  | [NOTICE] 094/155107 (1) : New worker #1 (8) forked
app-4      | MyWebServer: You can connect to your server at http://localhost:8080/
app-2      | MyWebServer: You can connect to your server at http://localhost:8080/
app-5      | MyWebServer: You can connect to your server at http://localhost:8080/
app-9      | MyWebServer: You can connect to your server at http://localhost:8080/
app-7      | MyWebServer: You can connect to your server at http://localhost:8080/
app-6      | MyWebServer: You can connect to your server at http://localhost:8080/
app-1      | MyWebServer: You can connect to your server at http://localhost:8080/
app-10     | MyWebServer: You can connect to your server at http://localhost:8080/
app-8      | MyWebServer: You can connect to your server at http://localhost:8080/
app-3      | d3757cd065b4
app-3      | d3757cd065b4
app-3      | d3757cd065b4
app-4      | 226419df4354
app-4      | 226419df4354
app-4      | 226419df4354
...

More details here.

What am I missing?