I am using Dietpi rather than ARCHLINUX.
I have a USB drive used for Roon backups mounted at /mnt/USB_NVME_256
This drive already has a RoonBackups folder in it because it is one of my scheduled backup locations.
If I perform a manual backup with the destination set to /mnt/USB_NVME_256, then the backup works fine and actually performs the backup to /mnt/USB_NVME_256/RoonBackups.
However, if I specify /mnt/USB_NVME_256 as the destination of a scheduled backup, things start to go wrong. It says it performs the backup successfully but the backup is not visible within Roon. On the disk I see that, instead of putting the backup in the RoonBackups folder, it has simply created a folder with a GUID style name (xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx where x is a hex digit - 0 to 9, a to f) and internally that folder looks like a Roon Database backup.
Conversely, if I explicitly set the scheduled backup location to /mnt/USB_NVME_256/RoonBackups, then the backup works. It is put in the specified folder and it can be found within Roon both using the find backups method and the scheduled backup view - clean/restore method.