yes, AMD Ryzen 5 5600X. I also found that large and hard to follow kernel bug thread, and found another similar issue/thread reported here @ Ryzen mce Issue at Boot
I was somewhat glad to read in your thread that this happens very infrequently for you, considering some people reported very frequent issues. I quite honestly hope this first occurrence for me is my last.
One of the comments in the kernel thread suggested some 3000 series CPU users felt changes AMD made for the 5000 series (my series) introduced the problem… but who knows? With so much packed into a constantly evolving kernel, it’s not outside of the realm of possibility there is a hard to nail-down issue in flight.
Did you happen to read the (year old) attached “possible fix” diff on that kernel bug @ https://bugzilla.kernel.org/attachment.cgi?id=290035&action=diff ? Found it interesting that the focus was on AMDGPU… and there have been a lot of ongoing AMDGPU changes.