Thanks for your reply.
docker-compose down , docker volume rm
mailcowdockerized_rspamd-vol-1 and the volume got deleted successfully. The Web UI was empty, as expected.
docker-compose exec rspamd-mailcow rspamadm dmarc_report. It did generate the Reports for two days ago. Upon executing that same command again, it said the same thing:
Process date 20220819
No reports for 20220819
send data for 0 domains (from 1 to 0)
Same thing. Web UI date is right, date for report is wrong.
Well, for the deleting the volume, it does kind of delete the data. The start page of the Web UI is empty but the history and learns are still there.