IPv6 address scope not set to operator-configured value

[Tj <linux@iam.tj>]

Using iproute2 and kernel v6.5.0 with Debian 12 Bookworm amd64 (tested also with v6.136 nixos) setting scope on an IPv6 fails silently with no indications as to why and the address is configured with what appears to be a scope based on the prefix (usually 0 but for fe80::/16 addresses scope is set to 253). Doesn't matter whether using scope names (from /etc/iproute2/rt_scopes) or numbers. Similar command for IPv4 succeeds.

ip address add fddc::2/64 scope 200 dev PUBLIC
ip -N -6 address show dev PUBLIC
...
inet6 fddc::2/64 scope 0

I used `gdb` to trace this expecting somehow the scope was not being read correctly but it is:

2577            if (!scoped && cmd != RTM_DELADDR)
(gdb) p scoped
$22 = <optimized out>
(gdb) p cmd
$23 = <optimized out>
(gdb) n
2580            req.ifa.ifa_index = ll_name_to_index(d);
(gdb) p req.ifa.ifa_scope
$24 = 200 '\310'
...
2607            if (echo_request)
(gdb) n
2610                    ret = rtnl_talk(&rth, &req.n, NULL);
(gdb) p req.n
$25 = {nlmsg_len = 64, nlmsg_type = 20, nlmsg_flags = 1537, nlmsg_seq = 0, nlmsg_pid = 0}
(gdb) p rth
$26 = {fd = 3, local = {nl_family = 16, nl_pad = 0, nl_pid = 2381950, nl_groups = 0}, peer = {nl_family = 0, nl_pad = 0, nl_pid = 0, nl_groups = 0}, seq = 1694191286,
   dump = 0, proto = 0, dump_fp = 0x0, flags = 4}
(gdb) s
rtnl_talk (rtnl=0x5555555f7020 <rth>, n=n@entry=0x7fffffffe140, answer=answer@entry=0x0) at ./lib/libnetlink.c:1170
1170    {
...
ipaddr_modify (cmd=<optimized out>, flags=<optimized out>, argc=<optimized out>, argv=0x7fffffffe478) at ./ip/ipaddress.c:2612
2612            if (ret)
(gdb) p ret
$27 = 0