a question for feeds (especially for custom event provider)

gistfile1.txt

// identity.py
def main(args):
    print(args)
    return { 'args' : args }

$ wsk package create feedtest --shared=yes
ok: created package feedtest

$ wsk action create feedtest/identity identity.py --kind python:3
ok: created action feedtest/identity

$ wsk trigger create T_id --feed feedtest/identity
ok: invoked /_/feedtest/identity with id f962b8694daf4f01a2b8694daf1f019a
{
    "activationId": "f962b8694daf4f01a2b8694daf1f019a",
    "annotations": [
        {
            "key": "path",
            "value": "jin.c/feedtest/identity"
        },
        {
            "key": "waitTime",
            "value": 766
        },
        {
            "key": "kind",
            "value": "python:3"
        },
        {
            "key": "limits",
            "value": {
                "logs": 10,
                "memory": 256,
                "timeout": 60000
            }
        },
        {
            "key": "initTime",
            "value": 404
        }
    ],
    "duration": 406,
    "end": 1530072881286,
    "logs": [],
    "name": "identity",
    "namespace": "jin.c",
    "publish": false,
    "response": {
        "result": {
            "args": {
                "authKey": "2b5160f8-a124-4fab-b22a-9620e9c7462e:oCvRZRDZMzb4l1bkSQ9LCxW0WkCJitXox9IN3zFfjjlA1TnpGfvYjtN02ioAEZav",
                "lifecycleEvent": "CREATE",
                "triggerName": "/_/T_id"
            }
        },
        "status": "success",
        "success": true
    },
    "start": 1530072880880,
    "subject": "jin.c:jin.c",
    "version": "0.0.1"
}
ok: created trigger T_id

$ wsk list
Entities in namespace: default
packages
/jin.c/feedtest                                                        shared
actions
/jin.c/feedtest/identity                                               private python:3
triggers
/jin.c/T_id                                                            private

// when T_id is fired no activation id is produced.
$ wsk trigger fire T_id
ok: triggered /_/T_id with id 

// No activation id is produced by firing T_id trigger, either.
$ curl -X POST -H "Content-Type: application/json" https://2b5160f8-a124-4fab-b22a-9620e9c7462e:oCvRZRDZMzb4l1bkSQ9LCxW0WkCJitXox9IN3zFfjjlA1TnpGfvYjtN02ioAEZav@frisb.kakao.com/api/v1/namespaces/_/triggers/T_id