Dear maintainers,

I am reporting a progressive failure of the cgroup2 hierarchy on Debian 12
when using kernel 6.12.74-2~bpo12+1 (bookworm-backports).

After some uptime, system and user systemd services begin failing with:

Failed to create cgroup ...: No such file or directory
status=219/CGROUP

Once triggered, systemd can no longer create new cgroups, which causes
continuous service restart loops and makes the system unusable until
reboot. Reverting to kernel 6.1.x resolves the issue under the same
workload.

Environment:
- Debian 12 (bookworm)
- systemd 252
- unified cgroup v2 hierarchy

Additional notes:
- Affects both system.slice and user.slice.
- The issue is not immediate at boot.
- No OOM or oomd activity has been observed.
- The problem appears only with kernel 6.12.74-2~bpo12+1 and not with 6.1.x.

Best regards,
Pedro Castro