Rally op_metrics throughput is null

[hyt]

es: 7.17.9
rally: 2.6.0 and 2.8.0
python: 3.8
cmd:
esrally race --track=http_logs --target-hosts=172.20.236.173:9200 --pipeline=benchmark-only --kill-running-processes --track-params=rally_params.json --offline
track：http_logs
Anomalous phenomenon :
index-append client is 8 , everything is normal
index-append client is 16，the result of index-append is null
{
"task": "index-append",
"operation": "index-append",
"throughput": {
"min": null,
"mean": null,
"median": null,
"max": null,
"unit": "docs/s"
},
"latency": {},
"service_time": {},
"processing_time": {},
"error_rate": 0.0,
"duration": 225703.49534900743
}