I had a problem with ipv6 myself with gmail, but the problem was that I just didn't have ipv6 fully set up on my server, so either SPF or reverse DNS wasn't working or something like that. I think I just configured sendmail to only use ipv4 and that solved the gmail issue.
My issue was that I wasn't very familiar with ipv6, and my ISP (OVH) apparently gave my server a range of about 256 ipv6 addresses, and I didn't really know how to properly set up reverse DNS and SPF. After spending a day or two getting nowhere, I just decided to turn off ipv6 completely for the server.
But gmail was not. Even as a business user with support.
Gmail rejects me as an ugly spammer at the gate when using IPv6 but not when using IPv4.
My IP addresses are not listed in any public blacklists.
And the Borg hivemind is not able to tell me why. Gmail support is friendly but have no knowledge of their filter nor an escalation path.
The amusing part is they reject me as a bulk sender. But when I register into their bulk mailer program my volume is too low.
This is with strict SPF, DKIM, DMARC and registered with dnswl.org.