On this forum it has been advised by many people that with IPv6 DHCP client you should not turn on “add-default-route” because it has always been a MikroTik hack. DHCPv6 does not have route information. An example of such posts:
http://forum.mikrotik.com/t/ipv6-configuration-under-router-os-7/170929/1
So, you should turn it off, and rely on accept-router-advertisements=yes under IPv6 settings (you already have this setting turned on). Otherwise, there might be a bogus default route in your route table if you keep “add-default-route=yes”.
What does
/ipv6 route print
list as default route (dst-address ::/0) if you keep the current setting?