fwd 2001:470:6e:525::1 proto ip6 src-ip6 2001:470:6f:525::/64
if you change in this rule "ip6" -> "ipv6" which should be according to man page possible no packets are matched and no error is reported.
According to the man page: "The ipv6 in proto option will be treated as inner protocol." ie ip6 denotes outer protocol. This is only relevant where there is tunnelling. and it's debatable how this should be treated where there is no tunnelling.
(In reply to Bob Bishop from comment #1)
Indeed, `ip6` is the reserved keyword, matches all upper level protocols that are used within IPv6. `proto ipv6` is protocol 41, means IPv6 datagram encapsulated inside.