That dad-gummed lazy sysadmin (me) had not updated any software, kernel or otherwise, in weeks. (at time of failure)
auto-update disabled and no manual update for nearly a month
After the last manual update did you restart the systems could there have been a delayed effect?
Otherwise it seems like a case of the X Files.
A software update shouldn't fry the Ethernet adapters should it?
I'd try another OS to see what happens. NetBSD is very low-level; in my biased opinion, I'd say it's very suited to this.
I'm seeing the same issue now with my Pine64 that I dug out of a closet. It had Armbian 16.04 running on it and network working as usual. After upgrading to latest Armbian Focal eth0 is gone and I get the same reported dmesg message.