Why does odhcpd halve the preferred leasetime of dhcpv6 leases?

When using systemd-networkd to request a static ipv6 address from from an openwrt router. I run into the problem illustrated by the following trace. The address to follow is IA_ADDR 2001:8b0:1418:4c2f::9 pltime:2372 vltime:2372

34 dhcp6 renew (xid=1c3ff1 (server-ID hwaddr type 1 c25627d4a752) (IA_NA IAID:9 T1:1186 T2:1897 (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295) (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:2372 vltime:2372)) (option-request DNS-server DNS-search-list NTP-server SNTP-servers) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (elapsed-time 0))
35 dhcp6 reply (xid=1c3ff1 (server-ID hwaddr type 1 c25627d4a752) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (opt_82) (DNS-server fda0:4d7f:afbb:4c2f::1) (IA_NA IAID:9 T1:580 T2:928 (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:1161 vltime:1161) (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295)))

42 dhcp6 renew (xid=973c61 (server-ID hwaddr type 1 c25627d4a752) (IA_NA IAID:9 T1:580 T2:928 (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295) (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:1161 vltime:1161)) (option-request DNS-server DNS-search-list NTP-server SNTP-servers) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (elapsed-time 0))
43 dhcp6 reply (xid=973c61 (server-ID hwaddr type 1 c25627d4a752) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (opt_82) (DNS-server fda0:4d7f:afbb:4c2f::1) (IA_NA IAID:9 T1:285 T2:456 (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:571 vltime:571) (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295)))

52 dhcp6 renew (xid=4f072e (server-ID hwaddr type 1 c25627d4a752) (IA_NA IAID:9 T1:285 T2:456 (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295) (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:571 vltime:571)) (option-request DNS-server DNS-search-list NTP-server SNTP-servers) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (elapsed-time 0))
53 dhcp6 reply (xid=4f072e (server-ID hwaddr type 1 c25627d4a752) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (opt_82) (DNS-server fda0:4d7f:afbb:4c2f::1) (IA_NA IAID:9 T1:130 T2:208 (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:261 vltime:261) (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295)))

and so on until

274: 106 01:45:01.745699 c2:56:27:d4:a7:52 > b8:27:eb:d4:eb:54, ethertype IPv6 (0x86dd), length 198: (flowlabel 0xc541e, hlim 64, next-header UDP (17) payload length: 144) fe80::c056:27ff:fed4:a752.547 > fe80::ba27:ebff:fed4:eb54.546: [udp sum ok] dhcp6 reply (xid=3eeea5 (server-ID hwaddr type 1 c25627d4a752) (client-ID hwaddr/time type 1 time 558114890 b827eb719903) (opt_82) (DNS-server fda0:4d7f:afbb:4c2f::1) (IA_NA IAID:9 T1:4294967295 T2:4294967295 (IA_ADDR fda0:4d7f:afbb:4c2f::9 pltime:4294967295 vltime:4294967295) (IA_ADDR 2001:8b0:1418:4c2f::9 pltime:0 vltime:0)

At which point the client gives up and removes the address from the interface and does not attempt to reaccquire it.

I can get round this by having the client request an infinite lease. But I would prefer not to.

This does not look like the correct behaviour to me. Leasetime half life is a matter for the client not the server.

Comments anyone?