Ubuntu 26.04 LTS install fails on Ryzen 9950X3D — possibly LP #2143762?

I want to install Ubuntu 26.04 LTS on my new build but the installer consistently fails 3 seconds into the file copy phase. I've ruled out all the typical USB/ISO causes — listing them upfront so we don't go in circles.

Hardware:

- AMD Ryzen 9 9950X3D (Zen 5)
- MSI MPG X870E CARBON WIFI (BIOS 1.A99)
- NVIDIA RTX 4070 SUPER
- 2x NVMe Samsung
- Secure Boot enabled, UEFI only

Error (literal from subiquity log):

Apr 30 20:51:02 ubuntu subiquity_log: Installing sources:
[{'type': 'tgz', 'uri': 'cp:///tmp/tmpf07u6hus/mount'}]
to target at /target

Apr 30 20:51:02 ubuntu subiquity_log: Running command
['rsync', '-aXHAS', '--one-file-system',
'/tmp/tmpf07u6hus/mount/', '.']
with allowed return codes [0]

Apr 30 20:51:05 ubuntu subiquity_log:
rsync: connection unexpectedly closed
(0 bytes received so far) [Receiver]

Apr 30 20:51:05 ubuntu subiquity_log:
rsync error: error in rsync protocol data stream
(code 12) at io.c(232) [Receiver=3.4.1]

Already ruled out:

- ISO SHA256 verified against the published checksum on releases.ubuntu.com
- USB written with `dd` from Ubuntu 25.10 (one attempt)
- USB also written with Rufus in dd mode from Windows (second attempt, different tool)
- USB scanned for bad sectors — clean
- Same USB installs Fedora 44 successfully on the same hardware

The medium is fine, the file is fine as served by Canonical, and the hardware is fine. The corruption seems to be inside the ISO itself.

Possibly related:

Launchpad #2143762 — Michael Hudson-Doyle reproduced an analogous issue by adding `-cpu EPYC-Rome-v1` to a QEMU/lxd VM building Ubuntu Studio, suggesting xz/ifunc behavior on certain AMD microarchitectures corrupts the squashfs during ISO construction. The fix landed in `livecd-rootfs` on March 12, 2026:

> Backport "mksquashfs: don't create duplicate virtual -> real disk mappings" which causes corrupt squashfs files to be built (...)

That bug was tagged against Ubuntu Studio specifically, but the failure mode (rsync code 12, 0 bytes received from `cp:///` source) looks identical to mine on Desktop.

Questions:

1. Has anyone successfully installed Ubuntu 26.04 LTS Desktop on Zen 5 (Ryzen 7000X3D / 9000 / 9000X3D) since the April 23 release?
2. Is the build-host fix from #2143762 actually applied to the Desktop ISO currently served on releases.ubuntu.com, or was it only Studio?
3. Has anyone hit code-12 rsync at the same point and resolved it?
4. Is there a daily build / point release that's known good?

I'm currently on Fedora 44 because of this, but I'd really prefer to be on Ubuntu LTS (signed shim, no manual MOK enrollment for NVIDIA drivers, 5-year support cycle).

Tried the fallback of installing 25.10 first and `do-release-upgrade` to 26.04, but Canonical's infrastructure was unavailable that day, so that path was closed too.