Default IPv6 firewall rule impacts recent Comcast Xfinity change.
http://forum.mikrotik.com/t/xfinity-comcast-dhcpv6-configuration-change/156031/7 is worth reading.
Default setup for Router Advertisements and Forwarding interact and affect default gateway acquisition.
http://forum.mikrotik.com/t/possible-bug-with-dhcpv6-in-routeros-7-8/164906/1 is worth reading.