If your IPv6 default gateway is set to aaaa:bbbb:cccc:1234:: the default gateway is not configured in the routing table, even though the entry is correctly set in /boot/config/network.cfg. The solution is to use aaaab:bbbb:cccc:1234::0 which works, and ironically then displays as the next hop aaaa:bbbb:cccc:dd00:: in the routing table. Bit of a gotcha there.