The problem is mainly that IPv4 port forwarding is network address translation, but on IPv6 it’s instead IP forwarding with a firewall rule.
The latter is conceptually simpler, but it’s a different mechanism and one that most home routers don’t bother to implement. This is quite ironic because IPv6 was intended to restore end to end connectivity principles.
Don’t get me wrong; I’m quite happy with the standard. They are very few good implementations of that standard, and given the momentum of its predecessor, implementers just don’t care.