My top concern in raising this is security: did I do something that compromised my DNS or was PiHole upstream compromised? At this point, neither are likely, bordering on impossible, so that's out.
My next concern was misconfiguration: did I do something that was causing this? Unlikely, because, frankly, I've not done anything to the PiHole installation aside from my custom logging setup. So, that's out.
My next goal was reporting to see if anyone else had encountered this and enable devs to help troubleshoot what could have caused it. It looks like we're all at ¯\_(ツ)_/¯ stage with it, so…
At this point, I think my effort on this is best expended simply redeploying a fresh PiHole setup from scratch on a fresh installation of a the latest stable Debian, reintegrating my logging solution, and moving on with life! We'll chalk this one up to a weird occurrence and see what happens next.