DHCPv6 PD lifetime on Japan NTT 10G Hikari

[missing233]

I'm using OpenWRT with NTT 10G Hikari (Flets Cross). I've encountered a specific issue regarding the DHCPv6 PD.
From the NTT DHCPv6 server, my DHCPv6 Client has been assigned a /56 PD prefix with the following lifetimes:
T1: 7200
T2: 10800
Preferred Lifetime: 12600
Valid Lifetime: 14400

The problem manifests itself at specific intervals — typically at the 4-hour, 6-hour, and most notably, the 8-hour marks. Just when NTT's DHCPv6 server sends out a reconfigure message, within about a second, I get a notice: 'daemon.notice netifd: Interface 'wan6' has lost the connection'. The connection will restore in about 10 seconds.
My ISP uses an IPv4 over IPv6 protocol, requiring a special IPv6 address as a tunnel endpoint. This issue only pops up when I assign this particular IPv6 address to the IPv4 over IPv6 tunnel interface (by using option ip6ifaceid).

The ISP provided me with a router XG-100NE, and I've also done packet capture analysis on it. I've noticed that each Renew Message behaves differently from OpenWRT. Could this be the cause?

OpenWRT:
Identity Association for Prefix Delegation
Option: Identity Association for Prefix Delegation (25)
Length: 41
IAID: 00000001
T1: 0
T2: 0
IA Prefix
Option: IA Prefix (26)
Length: 25
Preferred lifetime: 0
Valid lifetime: 0
Prefix length: 56
Prefix address: 2400:xxxx:xxxx:xx00::

ISP router:
Identity Association for Prefix Delegation
Option: Identity Association for Prefix Delegation (25)
Length: 41
IAID: 00000000
T1: 7200
T2: 10800
IA Prefix
Option: IA Prefix (26)
Length: 25
Preferred lifetime: 12600
Valid lifetime: 14400
Prefix length: 56
Prefix address: 2400:xxxx:xxxx:xx00::

[missing233]

Alright, I've set up a crontab scheduled task to make odhcp6c send Renew Message to update the IPv6 lifetime every hour.
It seems to work, but I don't think it is a good solution.

[Headcrabed]

@dedeckeh can you have a look at this? More information here: openwrt/openwrt#13454