This seems interesting; according to [RFC 5006](https://tools.ietf.org/html/rfc5006#section-1.2):
>If RDNSS option is included in the RA messages, an IPv6 host may perform DNS configuration through DHCPv6 [6]-[8] regardless of whether the O flag is set or not.

RFC 6106 which obsoletes 5006 does not mention this though...