Of course it is the NAT. It always was the NAT. NAT is the easy thing, nearly effortless, that dramatically improves network security — there is a "bastion host" (router) and private network computers, almost impossible to be accessed from the outside without breaking the bastion host first. This is good security, and with NAT, everybody was getting it for free. With IPv6, it doesn't work this way, everybody needs to…
Also, all consumer routers simply block any incoming IPV6 packets, this has been the default for more then 10 years now. (the only thing not blocked is ICMP for MTU path-discovery, which is actually a good thing).