I hadn't done anything to any of the routers, but all of my servers, workstations, and compatible devices have IPv6 addresses again.
I didn't recall the settings to return them, so I'm not sure if the settings were changed or entered by someone else (ISP, maybe), but while trying to debug a CDN problem (related to the fact that my edge reverse-proxy can't do SSL yet), I noticed tons of IPv6 in the logs, and in my debugging attempts.
I guess it's a good thing. The firewalls were configured, and few services are bound to any IPv6 addresses, so as far as I know the machines are still safe.
Something to work on now, I guess.