So after a lot of digging, I stumbled upon a post that mentioned 'fast boot' could cause an issue. I do not have fast boot enabled, but it gave me the idea it would be worth trying to shutdown, disconnect the power cord, wait, and try again. This appears to have resolved the issue!
I'll post back in a couple days to verify all's well (or before if all isn't), so the thread can be closed.
For now, would still be interested in any insight on what might have happened though
I have no idea, i am jusing Fastboot without issue on a Ryzen and Threadripper system. Glad you sorted it though!