Auto-configuration IPv6 settings on WAN page
The autoconfig option on WAN interface should be marked as "SLAAC/DHCPv6" rather then "DHCPv6".
It is possible to have auto-configuration without DHCPv6 IA_NA (M=0 in RA). On the other hand SLAAC is mandatory for auto-configuration because of routing. IPv6 educated user might look for the SLAAC option.
Note: DHCPv6-PD is not dependent on any of the IPv6 WAN option because CPE MUST use it (RFC 6204 WPD-4).