Unfortunately, https://forum.opnsense.org/index.php?msg=262791 seems to apply to 26.1 as much as to the 25 series. I have now tried to switch from dnsmasq (which has another bug with SAs, to be reported separately) back to radvd and KEA for DHCPv6, plugging in a carp.d hook script to only let radvd run on the respective master.
This seems stable for my desktop devices, but Android devices, though they consistently get a SLAAC pair of addresses, fail to connect.
Is https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=280701 supposed to apply to 26.1 kernels? Has the whole patch set been reverted or is it supposed to be fixed?