For whatever reason, zen.spamhaus.org starts to block entire IPv6 ranges/net blocks and their website doesn't show any mention of that. I can see it due to errors arriving to my servers when they try to email each other.
I then tested some of my IPv6 address via their website at https://www.spamhaus.org/lookup/
and all came back as listed. Even for IPv6 addresses that are not attached to anything. Interestingly, the equivalent IPv4 address are not listed.
About an hour ago, I got de-listed.
Its good that my postfix marks zen.spamhaus.org with few points, so nothing is being blocked right away, but its sad to see such a good service fall to such horrible practices.