After systemd upgrade, why does /dev/null always have the wrong permissions?

After upgrading my Debian system, this happens:

# ls -l /dev/null 
crw-rw-r-- 1 root plugdev 1, 3 Feb 20 18:17 /dev/null

...which is bad. I can manually change the permissions, but it doesn't persist across reboots, of course. Most things work, but given that most (but not all) of the other device nodes are owned by plugdev, I think something's seriously wrong and it's only working by accident.

I suspect that what's happened is a borked upgrade to systemd. However, I barely understood udev, let alone systemd.

I can't find anything in the logs about initialising the device nodes. Nor can I find anything in the documentation. Nor can I find any udev rule files or systemd unit files which do this.

Has anyone seen this before, and if so, what might be going wrong? And if nobody has, where should I be looking to diagnose this?

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