Just to be sure, I ran memtest86+ for 15 hours, and tried Mint and Fedora live environments. No reboots in multiple shutdown attempts. Arch, fix your *#$%! 
Yup, that did the trick. Didn’t even know that was required, since I’ve only played with kernel parameters when it comes to GRUB. 
P.S. And no reboots when poweroff is given through there, either.
Update:
I’ve had two successful shutdowns from the operating system since today’s updates. I do not know if it’s something in kernel 6.6.4-6, or some other package, but things seem to be back to normal 
Edit:
0001-drm-amd-Put-dGPUs-into-D3cold-when-shutting-down-the.patch
That did seem related in file name, at least. Patch has now been removed.