Rally error when bulk-size=1

I'm using Rally to simulate an Elasticsearch single node receiving logs on by one.

The following parameter of track.json:

        "bulk-size": 1,
        "ingest-percentage": {{ingest_percentage | default(100.0)}}

Should make Elasticsearch index 100% of the docs one by one. (If I understand it right)
Except that when Rally run the bulk, it's slowy getting done (normal)
but it get stuck at

Running bulk                                                                   [100% done]

Any idea why it get stuck?

Can you please share ~/.rally/logs/rally.log and ~/.rally/logs/actor-system-internal.log?

There is no ~/.rally/logs/actor-system-internal.log file

Then just rally.log please! If it contains sensitive information I can share an upload link.