The destination index is not updated by continuous transform

Elastic Stack Elasticsearch
1

Hi Team, I have created a transform by api as follows:

PUT /_transform/product_transform
{
  "source": {
    "index": "all_requests"
  },
  "dest": {
    "index": "product_transform_index"
  },
  "pivot": {
    "group_by": {
      "product": {
        "terms": {
          "field": "product"
        }
      }
    },
    "aggregations": {
      "action_count": {
        "value_count": {
          "field": "action"
        }
      },
      "timestamp": {
        "max": {
          "field": "request_time"
        }
      }
    }
  },
  "frequency": "2s",
  "sync": {
    "time": {
      "field": "request_time",
      "delay": "1s"
    }
  }
}

and exec POST /_transform/product_transform/_start, but the destination index is not updated on time and keep the original data forever.

source index: all_requests, created script is as following:

PUT /all_requests
{
  "settings": {
    "number_of_shards": 1,
    "number_of_replicas": 1
  },
  "mappings": {
    "properties": {
      "product": { "type": "keyword" },
      "request_time": { "type": "date" },
      "response_time": { "type": "date" },
      "action": { "type": "keyword" }
    }
  }
}

ES&Kibana: 8.5.1
Could you help me to solve this issue? Thanks.