DHCP - DHCPv6 and the default gateway
DHCPv6 is the version of DHCP that is used with IPv6. DHCPv6 does not typically set the default gateway for IPv6 clients. The default gateway is typically distributed through Router Advertisements (RAs) sent by IPv6 routers as part of the Neighbor Discovery Protocol (NDP), not through DHCPv6 itself.
Indeed, DHCPv6 has no standard option specifically designed for directly assigning the default gateway to clients in the same way that it does for IPv4. This is a significant difference from IPv4, where the default gateway is typically distributed via DHCP.
The default gateway on the clients is determined based on the RA messages received from the local IPv6 router, which will include the default gateway information.
Links:
https://forum.networklessons.com/t/cisco-dhcpv6-server-configuration/1116/89?u=lagapidis
https://networklessons.com/ipv6/cisco-dhcpv6-server-configuration/