Continuous transform is "indexing", but no new destination records

Elastic Stack Elasticsearch
1

My last record in the destination index is for the minute, 2020-08-11 20:05:00.

It is a continuous transform and it's status=indexing, but no new destination logs are being generated.

I have other transforms in continuous mode; I don't know why this one is failing.

see messages below... and stats attached

2020-08-11 20:05:42
df-elastic-es-data-nodes-1
Finished indexing for transform checkpoint [0].
2020-08-12 03:23:28
df-elastic-es-data-nodes-1
Finished indexing for transform checkpoint [1].

Screenshot 2020-08-12 at 10.26.58
Screenshot 2020-08-12 at 10.26.581746×1030 122 KB

2

hmmmm stopping it and starting it again has got it outputting more destination logs again, but that defeats the point a bit!

Do I have a conflict here?
recv_timestamp is the originating source time of the msg
ingest_timestamp is added by Elasticsearch to the index at the time the doc is indexed

  "frequency": "1s",
  "sync": {
    "time": {
      "field": "ingest_timestamp",
      "delay": "10s"
    }
  },
  "pivot": {
    "group_by": {
      "recv_timestamp": {
        "date_histogram": {
          "field": "recv_timestamp",
          "calendar_interval": "1m"
        }
      },
3

and after the restart, it only reached the first checkpoint.
so I've stopped and restarted again, and I expect it will pause at the next checkpoint again

4

I still have a problem here.
The status will be indexing, but no data is coming out. Then after a few hours there are a splurge of new destination documents output