This is only true when
- you installed CachyOS from a “newer” ISO (January 2026 for example) or did the following things manually
- the drive in question is formatted with btrfs
- you use Limine as bootloader
- the limine-snapper-sync magic is working correctly
- Snapper is set up correctly (which, as we currently believe, is not the case)
We do not know if OP fulfills all these prerequisites, so I’d be careful with such statements.
Back to topic: I suspect a problem with either sddm or plasma-login-manager, but as I have no idea how to verify (let alone mitigate) that, I better keep shut 