I have the same issue on RUTX12. Some tshooting gives me an information that dchpv6 is leaking memory. When I enable server mode or hybrid mode, the router reboots ones in a couple of hours (2-3 hour). So, if you config stateless IPs and router reboots all active leases info gone and IPv6 stop working.... The workaround use statefull IP. And please do something with dhcpv6.