Silence user via Admin API can't setup reason string

I had follow the API doc to make the silence request via Admin SDK, it is working, the user got silenced but didn’t change the reason string for silence.

The request:

PUT: /admin/users/{id}/silence
Payload: {
 "message": "silence",
 "silenced_till": "2021-06-22",
 "reason": "Reason message appears empty",
 "post_action": "delete"
}