Docker Swarm mode running many services with healthcheck eventhough replicas=1

Question

I have a single node with Docker in Swarm mode. I deploy a stack with a service with this config:

    deploy:
      mode: replicated
      replicas: 1
      update_config:
        parallelism: 1
        order: start-first

I hoped it deploys an instance first, before taking down the old one. But to make it only remove the old one when the new one is ready to receive traffic I add a healthcheck. Then instead of one new starting container, suddenly I get 5 starting containers for the service at the same time and the server is almost breaking under the load. Why are there so many instances when I only enable a healthcheck?