Thank you for the suggestion about changing the firewall.
Comcast / Xfinity's recent network upgrade in my neighborhood caused me to run into this very issue!
I'm not sure what was actually necessary, but I changed the Firewall - Traffic Rule:
Allow-DHCPv6 + Allow Source Address ::/0 + Allow Destination Address ::/0
Alternately the source and destination IPs could have been deleted to make it a port rule only, but I wanted them left as comments to remind me what I changed.