This is unfortunately NOT a good change!
My ISPs only give out dynamic IPv6 prefixes via DHCPv6 which change with each PPPoE redial/router reboot. Those prefixes have both Valid Lifetime and Preferred Lifetime set at 5 hours. The problem is that when the router reboots for example, RouterOS doesn't announce that the old prefixes are deprecated, which means after a reboot the clients all have multiple prefixes, both new as well as the old, no longer valid ones, marked as preferred. As a result, they may have no IPv6 connectivity to the internet if they pick the addresses with the old prefixes as source addresses.
Until now I was able to workaround the issue by setting:
/ipv6 nd prefix default
set preferred-lifetime=10m valid-lifetime=10m
And the advertised prefixes are valid/preferred for at most 10 minutes. Which means when the issue occurred after a router reboot/redial, clients will have a period of invalid "preferred" addresses for at most 10 minutes, before the old addresses disappear and IPv6 WAN connectivity is possible again.
Now with the change in 7.21 the two settings above are ignored. Instead, the lifetime is now taken from the pool with the ISP prefix, and in my case with 5 hours lifetime.
I am now sitting here with my devices having no IPv6 for the next 4.3 hours:
Could you please change it, so that the lifetime values are the smaller values of the settings and what stored in the pool?