After finding a window for extended downtime I updated to 26.7.1_. Took several hours but came up just dandy in the end. So far so good. :)
However, now the SLAAC autogenerated address on the WAN IF is not the EUI-64 it used to be. It's still a stable address, but very much different from what the MAC-based version used to be. The latter still is in place on the link-local address of that interface, otherwise I'd have assumed that by some miracle the interface had changed its MAC, but it hasn't. I do have Privacy Extensions active but these generate another address that also changes properly and is listed as temporary just as it should be.
So, was there some change in SLAAC that now generates addresses in a different manner that I missed? I noticed there is the flag "STABLEADDR" in nd6 options=823<PERFORMNUD,ACCEPT_RTADV,AUTO_LINKLOCAL,STABLEADDR>, but IDK whether this was there before or not. So could this be a "stable privacy address" now? This matters for dynamic DNS, naturally.
It is. Stable privacy addresses are a new feature in FreeBSD 15.
Thanks for confirming! It seems that I had set net.inet6.ip6.use_stableaddr=1 in the tunables from some previous tests, forgot about it, and with the update it started to work. =) Now I can see if I can get that to play nicely with dynamic DNS, but ATM the updates seem to nor propagate at all. Probably I'll have to disable it on the Sensebox because I'm relying on the "dynamic IPv6 host" option already, and that won't auto-update on prefix changes (which it would need to do), but I'll need to wait and see how it actually fares. :)