IPv6 ULA Assignment Prefix Incorrect

Hi LEDErs,

I'm using LEDE as a router / firewall attached to my ADSL modem. My ISP supports DHCPv6-PD and I would like to assign IPv6 addresses to my subnet with both the global prefix and the ULA prefix, such that each client in my subnet has a consistent suffix (e.g. ::2, ::3, ..).

DHCPv6-PD, seems to be working fine, because clients on my subnet are readily picking up IPv6 addresses and getting routes, and the prefix seems to match with a randomized interface portion.

However, I would also like to ensure that my clients receive a ULA that is assigned by the LEDE box.

My IPv6 ULA is fd27:fcc0:fa2a::/48.

The LAN interface on my LEDE router (with ipv4 address) has the ULA fd27:fcc0:fa2a::1/64 (I'm not sure what the default mask is, but mine is set to 64).

I would like to keep this pattern going, i.e:
router fd27:fcc0:fa2a::1/64
node2 fd27:fcc0:fa2a::2/64
node3 fd27:fcc0:fa2a::3/64


However, I cannot seem to figure out how to get the LEDE DHCPv6 server to assign those addresses. They all get addresses like:

      inet6 addr: fd27:fcc0:fa2a:0:ba27:ebff:fed0:11d9%1994515404/64 Scope:Global
      inet6 addr: fe80::ba27:ebff:fed0:11d9%1994515404/64 Scope:Link

where the interface portion of the ULA is just a copy of the link-local address's lower 128 - N bits, where N is my prefix length.

Is this expected behaviour? Is the behaviour that I would like to see, with simple ULA::2, ULA::3, ULA::4, etc, somehow invalid?

My assumption was that I could configure the clients on my subnet to have easy-to-remember IPv6 ULA's was via LuCI under

Network -> DHCP and DNS -> Static Leases

where the IPv6 suffixes were my desired suffixes for IPv6 ULA's. That seemed to be the most intuitive way.

What is the correct way (ideally using LuCI) to configure IPv6 ULA's for my entire subnet, if the "Static Leases" section is incorrect?