web interface for gitlab runner does not show logging output

expected

I would expect to see the complete log output on web interface until my container execution is finished.

observation

web interface for gitlab runner stops showing logging output whenever there is a 150 seconds pause in logging output from the container.

The complete job takes ~6mins (including build from cache etc). My entrypoint written in bash in the main container stops writing anything to stdout/stderr for 150 seconds almost at the end of that process. When that script writes again to stdout/stderr I am missing the log on the "company".gitlab.com/.../jobs website. If I go onto the virtual machine and check with docker logs process00 I actually find the logs I would expect.

If I then add the following process to my entrypoint. I see a continuous logging including the complete log that I have been missing before on our website.

function heartBeat {
  TIMEOUT=0
  while [ true ] ; do
    sleep 5;
    TIMEOUT=$((TIMEOUT + 5))
    echo "### $(basename $0) has been alive for $TIMEOUT seconds"
  done
}

heartBeat &

more details

I run docker-compose with 4 containers in docker runner using a custom image. The same process runs on a shell runner does not cause that problem.

gitlab-ci.yml

test_pipeline:
    stage: second_stage
    image: $DOCKER_COMPOSE_IMAGE
    tags:
        - docker
    except:
        - schedule
    script:
        - docker-compose up --build --force-recreate --exit-code-from process00 2>&1
        - echo "test_pipeline done"
    cache: {}

gitlab-runner version

Version:      12.7.0
Git revision: 58272c27
Git branch:   12-7-stable
GO version:   go1.13.5
Built:        2020-01-20T08:32:35+0000
OS/Arch:      linux/amd64