I'm not sure how events are working in Supervisor but here are my thoughts:
even if there is the trigger of that event here, it is not dispatched, since events are buffered and there is no call to ProcessGroup.transition as done here.
The SUPERVISOR_STATE_CHANGE_STOPPING is not being triggered upon stopping the supervisor.
Example: sudo service supervisor stop.
Though I do get the "stop" event's indication in supervisord.log.
I'm not sure how events are working in Supervisor but here are my thoughts: even if there is the trigger of that event here, it is not dispatched, since events are buffered and there is no call to ProcessGroup.transition as done here.