My IPS changes my prefix once in while. I consider this a privacy feature, not a bug.
Now I find myself in a situation that my devices are not reachable anymore as when the IPv6 address changes and both DNS entries and firewall need to be updated each time when the prefix changed (In between connections break, but this might be a lesser problem)
As far as I understand the only solution which does not include some complex scripting of ip change detection and automatically updating the firewall rules is to use NAT66 and ULA. But even then I have a protocol whose most advertised feature is not to rely on NAT and puts mit fast in a situation in need to use NAT. And the privacy extension of every device or the devices using SLAC and not DHCPv6 are problematic.
IPv6 is just not able to steup efficiently for where IP addresses are changing. Not with moving mobile devices, not in wifi environments with multiple access points, not with changing prefixes, not in failover scenarios.
Bottom Line: I disabled IPv6 again here without any intentions to look for complex workarounds. All outbound traffic is now IPv4 again. IPv6 is providing no benefit but causes additional problems over IPv4 due to issues with the design. I am waiting for IPv7 (or whatever will be the next successor of IPv4) will arrive.