With the recent commits of odhcp now IPv6 relay mode seems to work
However LUCI is not consistent with this use case
Normally in this network setup you add a WAN6 as DHCPv6 client interface to get the upstream ipv6 on the router, and it is also likely you have the IPv4 WAN as a DHCP client.
Now you must set the master odhcp client on WAN interface (if you set on WAN6 it doesn't work for some reason):
config dhcp 'wan'
option interface 'wan'
option ignore '1'
option dhcpv6 'relay'
option ra 'relay'
option ndp 'relay'
option master '1'
But with WAN as DHCP client you can do it only by CLI/UCI, since in LUCI you don't get any ipv6 tab