steps:
    - name: first
      command: echo $DAG_RUN_STEP_NAME  

    - name: second
      stdout: "/tmp/dag_${DAG_RUN_STEP_NAME}_out.log"
      command: echo "meh"

• the first step outputs 'first' which is the step name as expected.

• the second step creates/updates a /tmp/dag__out.log
  the expected value is '/tmp/dag_second_out.log'

it does looks like a bug.