IPv6 addresses obtained by stateful DHCPv6 shows /128 prefix

Issue

• An IPv6 address obtained by a stateful DHCPv6 server shows /128 prefix.
  A different prefix, such as /64, is expected.

  $ ip a show eth0
  2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
      link/ether aa:aa:aa:aa:aa:aa brd ff:ff:ff:ff:ff:ff
      altname enp3s0
      inet6 2001:db8::aaaa/128 scope global noprefixroute <====================== /128 is shown 
         valid_lft forever preferred_lft forever
      inet6 fe80::aaaa:aaff:feaa:aaaa/64 scope link noprefixroute 
         valid_lft forever preferred_lft forever
  

• Stateful DHCPv6 is enabled in the environment.
  If it's an OpenStack environment, subnet settings is something like the following example.
  cidr is set to xxxx:xxxx::/64, not xxxx:xxxx::/128.

  $ openstack subnet show subnet_ipv6
  +----------------------+----------------------------------------------------------------+
  | Field                | Value                                                          |
  +----------------------+----------------------------------------------------------------+
     :
  | cidr                 | 2001:db8::/64                                                  |
     :
  | enable_dhcp          | True                                                           |
     :
  | ip_version           | 6                                                              |
  | ipv6_address_mode    | dhcpv6-stateful                                                |
  | ipv6_ra_mode         | dhcpv6-stateful                                                |
     :