At first I reported it in https://www.virtualmin.com/node/60457 but then I remembered I should have done it in the bug tracker, apologies.
I'll copy-paste what I wrote there, but just to be clear, it's not a plea for help, just a bug report.
I think I found a bug within virtualmin, but maybe that's actually normal and part of how the software ought to operate properly. So, well, I'm not sure, I come here to report it just in case.
Long story short: in Virtualmin admin > running backups, even if I toggle the currently running backup and successfully click to stop it, meanwhile, I will observe the tar creation process hasn't halted and is still ongoing even several minutes later.
How I found out: I made a silly mistake while creating a new scheduled backup, and asked virtualmin to backup over 150 GB of stuff (I totally screwed up, forgetting to exclude a directory where I store other stuff non-relevant to that website, I find it convenient to have an online ultimate resort backup location). Soon enough I realized I screwed up, so I went in the "running backups" submenu and successfully ended the backup. But on my server, 15 minutes later, the tarball is still being created, now in the 120 GB size and still growing every time I refresh the directory listing. And indeed, in the list of the running processes on the server, I found /bin/tar cfX - /var/tmp/webmintemp//40656_26341_5_backup.cgi --listed-incremental=/etc/webmin/virtual-server/incremental/13244744245671 .
I'll let the command line run to its end, it will be a close call but there'll be enough hard disk left, so I'm cool with it. But, still, I am not sure this is a "normal" behaviour, maybe that's a bug, and it's good that I report it then :)