Graceful restart helper is enabled by default when you start OSPFv3. You can disable and re-enable it.
device# configure terminal
device(config)# ipv6 router ospf
device(config-ipv6-router-ospf-vrf-default-vrf)# no graceful-restart helper
device(config-ipv6-router-ospf-vrf-default-vrf)# graceful-restart helper