Others and I have seen this issue before, I made this discovery too when I started using IPv6 in late October.
http://forum.mikrotik.com/t/problems-with-ipv6-on-bridge-interface/51310/3
In this topic
http://forum.mikrotik.com/t/ipv6-nd-not-working-with-interface-arp-mode-reply-only/51426/7
Normis / Janisk any progress on this issue?