Using latest drone version v1.2.3 with following pipeline:
kind: pipeline
name: otras condiciones
steps:
- name: by event
  image: alpine
  commands:
  - 'echo "Event: ${DRONE_BUILD_EVENT}"'
  when:
    event:
    - push
    - pull_request
    - tag
    - promote
    - rollback
It works perfect in push event but in tag do not work. Even more if you make a tag build and try to promote it the step is skipped too.