dnsmasq dhcp: Clients accumulating invalid IPv6 addresses when upstream changes

[fab]

Hello dear forum. I'm trying to migrate to the new dnsmasq DNS/DHCP server at the moment. But I have a strange problem, that if upstream [WAN] changes the delegated /56 prefix (when restarting the router for example), my WHOLE network accumulates these new addresses without invalidating the old defunct IPv6 addresses and the servers and workstations still try to use these invalid addresses, which of course ends with an error. And I can't test this without completely restarting my router. I still haven't found an option to trigger this dnsmasq functionality without restarting my router (sorry for being such a noob). It worked flawlessly with the old ISC dhcp server, and the old addresses were invalidated properly. I'm really frustrated, because I have no idea why this is happening. The only thing I can do if upstream [WLAN] disconnects (through a reboot of OpnSense), is restart all my servers and workstations, to get a good set of IPv6 addresses until [WLAN] goes down again.

And there's another problem (which many people seem to have according to google). On some machines there are still "valid" IPv6 addresses which have a lifetime of 24h and I can't get rid of them.

I can't give much of logs (there aren't many informative messages anyway), but I hope someone can help me anyway. But please be a little patient, I'm not dumb, but this kind of problem is completely new to me and IPv6 is really complicated. On one side I want the new functionality (if it worked) and on the other side my old setup with ISC dhcp worked as expected (I have 7 VLANs which worked flawlessly).

Thanks alot,
fab