Good to know you found the problem 
I think that mean the ISP uses DHCPv6 Relay or something like that internally. Probably a similar case to what customers of Xfinity/Comcast report.
http://forum.mikrotik.com/t/xfinity-comcast-dhcpv6-configuration-change/156031/1
That, in essence, means that the DHCPv6 Server address is not link-local and thus cannot be used as the default gateway.
I usually recommend to people to configure the DHCPv6 Client to add a default route (add-default-route=yes), but with a higher administrative distance (that equals to lower priority, like default-route-distance=3) than the default route obtained via Router Advertisement, which means it can still serve as a fallback in case the ISP messes up the timers in the Router Advertisements, for example. But in your case, since the DHCPv6 Server is not located in the same link-local space, you’re better off not adding the default route (add-default-route=no).