Your system information

• Steam client version (build number or date): steam-runtime_0.20220315.0
• Distribution (e.g. Ubuntu): Fedora Linux 35 (KDE Plasma) (64 bit)
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

The Restore function appears to be ignoring anything but the first part/disk/volume of a backup.

I believe this could be the same issue as described in:
https://github.com/ValveSoftware/steam-for-linux/issues/4762

I have experienced this issue come and go with diffrent Steam Client versions like the past 2 years, but as of now I think it has been like 6 months since this worked properly.

Steps for reproducing this issue:

• Make a backup of a large game and select default file size (CD 640 MB)
• Try to restore the backup of the game, observe how it only restores the first part/volume of the backup from local media

Real world example:

• Tried restoring a backup of the game "Alice Madness Returns App ID: 19680" backupped on a external drive (external USB connected HDD that is EXT4 formatted)
• The backup is a total of 6.9GB in 12 parts
• Restoring the game backup on Steam loads only the first part/disk from the local media, and then continues to download the rest of the game.

Workaround:
It is possible to workaround this issue by just using one part/volume/disk for the backups, by selecting a large "file size" for the backup. Or by manually copy/pasting the .csm and .csd files into the "Disk_1" folder and editing the sku.sis file to say "disks" "1"