This is due to the change introduced with 7.21:
From that version, the lifetime values you set in the default settings are ignored, and the lifetime is taken from the pool, which in turn gets the values from the ISP DHCPv6 lease (binding).
I've complained about this change in the RC thread, because the problem is even worse when the router is rebooted.
I've also suggested multiple times that they use the smaller value between the default that we set on the router and the value sent by the ISP as lifetime used by the advertisements, but alas, it was in vain.