Docker compose service with "network_mode: service: [...]" does not inherit parent's service networks

docker-compose-with-traefik.yml

version: "3.8"

networks:
  web:
    name: web

services:
  whoami:
    container_name: whoami
    image: containous/whoami:v1.5.0
    network_mode: service:net_holder
    labels:
      - traefik.enable=true
      - traefik.docker.network=web
      - traefik.http.routers.whomai.rule=Host(`whoami.EXAMPLE.TLD`)
      - traefik.http.services.whoami.loadbalancer.server.port=80
  whoami2:
    container_name: whoami2
    image: containous/whoami:v1.5.0
    networks:
      - web
    labels:
      - traefik.enable=true
      - traefik.docker.network=web
      - traefik.http.routers.whomai2.rule=Host(`whoami2.EXAMPLE.TLD`)
      - traefik.http.services.whoami2.loadbalancer.server.port=80
  net_holder:
    container_name: net_holder
    image: containous/whoami:v1.5.0
    command:
      - --port=9999 # We don't cara about the exposed port. This container just acts as a network holder
    # cap_add:
    #     - NET_ADMIN
    # devices:
    #     - /dev/net/tun
    networks:
      - web
  traefik:
    container_name: traefik
    image: traefik:2.2.1
    restart: unless-stopped
    command:
      - --api.dashboard=true
      - --log.level=INFO
      - --providers.docker=true
      - --providers.docker.network=web
      - --entryPoints.web.address=:80
    networks:
      - web
    ports:
      - 80:80
      - 443:443
    volumes:
      - /var/run/docker.sock:/var/run/docker.sock:ro
    labels:
      - traefik.enable=true
      - traefik.docker.network=web
      - traefik.http.routers.traefik.rule=Host(`traefik.EXAMPLE.TLD`)
      - traefik.http.routers.traefik.service=api@internal

docker-compose.yml

version: "3.8"

networks:
  web:
    name: web

services:
  whoami:
    container_name: whoami
    image: containous/whoami:v1.5.0
    network_mode: service:net_holder
  whoami2:
    container_name: whoami2
    image: containous/whoami:v1.5.0
    networks:
      - web
  net_holder:
    container_name: net_holder
    image: containous/whoami:v1.5.0
    command:
      - --port=9999 # We don't cara about the exposed port. This container just acts as a network holder
    # cap_add:
    #     - NET_ADMIN
    # devices:
    #     - /dev/net/tun
    networks:
      - web