Can not kill background process

Hello i am facing problem with gitlab-runner 11.7.0 (8bb608ff), (slef host gitlab)

• When run npm start will start up a web server with port 8080

• When run npm run build will build static files

• After npm run build success, the npm start process run backround but does not killed. so ci process will never stop

.yml file

image: "...."

# to cache both npm modules and Cypress binary we use environment variables
# to point at the folders we can list as paths in "cache" job settings
variables:
  npm_config_cache: "$CI_PROJECT_DIR/.npm"

# cache using branch name
# https://gitlab.com/help/ci/caching/index.md
cache:
  key: ${CI_COMMIT_REF_SLUG}
  paths:
    - .npm

test:
  stage: test
  script:
    - yarn install
    - npm run start &
    - npm run build

gitlab-runner 13.1.0 (6214287e) does not has this problem, can i fixed problem without upgrade?