DHCPv6 cannot provide a default route by protocol design. The “Add Default Route” setting in RouterOS is an unfortunately named hack and needs to be disabled unless you are told otherwise by the upstream administrator. Contact mikrotik support and tell them that this naming is extremely misleading.
In IPv6 gateway is discovered via Neighbor Discovery from Router Advertisement messages sent by the upstream router. Link-local address you see is perfectly fine for routing.
Do the following:
/ipv6/settings/set accept-router-advertisements=yes
/ipv6/dhcp-client/set add-default-route=no ether1
/ipv6/nd/add advertise-dns=no interface=ether1 ra-lifetime=none ra-preference=low reachable-time=5m
Make sure you didn’t configure your firewall to filter ICMPv6. You might need to reboot your router to avoid waiting for system timeouts.
You might want to check out http://forum.mikrotik.com/t/dhcpv6-client-cannot-recover-after-outage/167985/6 or search the forum for “xfinity”.