I have some strange issue with MTU on IPv6. I use /64 prefix from comcast. Getting some strange results from https://test-ipv6.com/. When I tun test on https://test-ipv6.com/ - i'm getting "Danger! IPv6 sorta works - however, large packets appear to fail, giving the appearance of a broken website. If a publisher publishes to IPv6, you will believe their web site to be broken. Ask your ISP about MTU issues; possibly with your tunnel. "
However, when I run IPv6 test against comcast-hosted test script - https://test-ipv6-vm4.comcast.net/ - It works without any issues. So, I believe the issue is not on my equipment. ICMPv6 allowed on firewalls, so shouldn't be an issue.
I think the issue is somewhere on the edge of comcast network in my region.
I'm not sure, if comcast engineers ever read this, but didn't find any other places to get support.