Comment 2 for bug 1962306

For workaround, what also worked for me to add gateway for subnet create (original command is in [1]):
$ openstack subnet create --ip-version 6 --ipv6-ra-mode slaac --ipv6-address-mode slaac --use-default-subnet-pool --gateway ::1 --network ipv6-pd ipv6-pd-1

[1]: https://docs.openstack.org/neutron/latest/admin/config-ipv6.html#user-workflow