Same problem here on my system:

## System Configuration

| Component | Details |
|---|---|
| **CPU** | AMD Ryzen 9 9950X3D (16 cores, 32 threads) |
| **Mainboard** | ASUS ProArt X870E-CREATOR WIFI (AMD X870E chipset) |
| **RAM** | 64 GB DDR5-6000 |
| **GPU** | ASUS Radeon RX 6600 Dual (AMD RDNA 2, open-source Mesa/amdgpu 
driver) |
| **Storage** | WD Black SN850X 2 TB NVMe (system), WD Black SN850X 4 TB NVMe 
(data) |
| **OS** | Ubuntu 26.04 LTS "Resolute Raccoon" |
| **Affected kernel** | 7.0.0-29-generic |

## Problem Description

After upgrading to kernel **7.0.0-29-generic**, both `shutdown` and
`reboot` hang after the systemd shutdown sequence completes
successfully. The monitors turn off, but the system remains powered on
indefinitely and must be hard-powered off.

## Symptoms

- `sudo systemctl poweroff` → systemd shutdown completes, monitors turn off, 
machine stays on
- `sudo systemctl reboot` → same behaviour, machine does not reboot
- `sudo systemctl poweroff --force` → same behaviour
- Hard power-off and restart required in all cases

## Kernel Parameters Tested (No Effect)

The following kernel parameters were added to
`GRUB_CMDLINE_LINUX_DEFAULT` individually and in combination — none
resolved the issue:

- `acpi=force`
- `reboot=acpi`
- `noapic`

## Shutdown Log (last entries before hang)

The systemd shutdown sequence completes without errors. The last
recorded log entries are:

```
systemd[1]: Reached target poweroff.target - System Power Off.
systemd[1]: Shutting down.
systemd-shutdown[1]: Syncing filesystems and block devices.
systemd-shutdown[1]: Sending SIGTERM to remaining processes...
systemd-journald[1391]: Received SIGTERM from PID 1 (systemd-shutdow).
systemd-udevd[1440]: Failed to remove file descriptor "config-serialization" 
from the store, ignoring: Connection refused
dnsmasq[5318]: exiting on receipt of SIGTERM
systemd-journald[1391]: Journal stopped
```

After `Journal stopped`, the system hangs indefinitely.

## ACPI Wakeup Sources

All wakeup sources are enabled on S4 (excerpt):

```
GPP3      S4    *enabled   pci:0000:00:01.4
GP17      S4    *enabled   pci:0000:00:08.1
XHC0      S4    *enabled   pci:0000:7c:00.3
XHC1      S4    *enabled   pci:0000:7c:00.4
XHC2      S4    *enabled   pci:0000:7d:00.0
GPP0      S4    *enabled   pci:0000:00:01.1
SWUS      S4    *enabled   pci:0000:01:00.0
SWDS      S4    *enabled   pci:0000:02:00.0
...
```

## Network

- Wake-on-LAN was enabled (`Wake-on: g`) on `enp13s0` — disabled as part
of troubleshooting, no effect on the issue

## Workaround (not fully 100% safe)

Boot permanently with kernel 7.0.0-28-generic by setting in
`/etc/default/grub`:

```
GRUB_DEFAULT="Advanced options for Ubuntu>Ubuntu, with Linux 7.0.0-28-generic"
```

Then run `sudo update-grub`.

Sometimes it works, sometimes not. :-(

-- 
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Bugs, which is subscribed to Ubuntu.
https://bugs.launchpad.net/bugs/2163015

Title:
  System fails to power off / hangs on shutdown-reboot after upgrade to
  Kubuntu 26.04 (kernel 7.0)

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