Unlike IPv4 DHCP offers, which contain the prefix and default route, IPv6 DHCP offers only contain the IP address assignment. The IPv6 route advertisement (RA) provides the default router and the prefix is set to /128 for the IP address supplied by the DHCP server.
For further information about RA support, see IPv6 DHCP/RA details. For further information about DHCP server offers, see DHCP server offer Options 66, 67, and 43.