For me, there is no modem - just an ONT on the wallIt was my modem. I factory reset my AX88U, added back settings manually, and still couldn't get IPv6 up and running. Today, I swapped out my CM1100 for a Moto MB8611, had Spectrum provision it, and bang, native IPv6 support was back.
No idea why Spectrum suddenly decided to block IPv6 on that particular modem model, but that's what it seems like they did.
I'll update if I lose IPv6 support again.