Bump.
The .tar and the final tar.gz both appear in /var/discourse/shared..../backup while it is running.
The tar.gz is visible in the web interface at /admin/backups while the uploads are being added to it (and it’s size increments up).
When it runs out of space the tar.gz disappears from /admin/backups, but the .tar file is still, and the space is not returned (this 4 hours after the backup failed).