CI shell runner not killing sub-processes

OS: Ubuntu 20.04
Runner: shell
Gitlab version: 13.9

We are using node.js and monorepo via lerna. Lerna build is spawning multiple processes in parallel for building, but they are not untied/un-refed from the main process AFAIK.

We have noticed, that there were no pipelines running, but there were still processes running in the runner node. It means that pipelines doesn't clear and doesn't kill all processes spawned/forked from it.

root@gitlab-runner:~# cat /etc/gitlab-runner/config.toml
concurrent = 25
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "gitlab-runner"
  url = "https://gitlab.example.com/"
  token = "<hidden>"
  executor = "shell"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]

With Docker, there wasn't such problems. Because it is killing everything related to current container.