Ugh. Thanks. It's quite possible, though maybe just a regional one? I did inadvertently block one of the IPs Let's Encrypt uses for secondary validation, so this may be another case of that.

I get a shitload of bad traffic from the southeast Asia area (mostly Philippines/Singapore AWS) and have taken to blanket blocking their whole routes rather than constantly playing whack-a-mole. Fail2ban only goes so far for case-by-case.

Here's the image from the meme from an alternate source: