Welp…Not sure what was wrong, but seems to now be resolving again. I…restored a few previous backups to no avail, rebooted to no avail…and after just giving up and upgrading, and rebooting…now it seems to work again. And I still have no idea how to troubleshoot this if it ever happens again :(

I have purposely disabled IPv6 everywhere. Router, Pihole etc. What do you mean which DNS am I using? The computer failing to resolve, or the Pihole that successfully resolves with dig, but somehow fails to actually resolve it to the pihole request/requesting computer?