I bet they get less reports on ipv6 issues than with ipv4. I have definitely seen carriers have ipv4 routing issues in enterprise networking.

I kinda think it may be on the isp then, maybe a v6 routing issue? You could work around with an ipv4 tunnel, then route ipv6 through it. If that’s possible, I’ve never done it haha.

How far does a traceroute get you? That could help isolate where the issue may be.