On 12.0-RC2 (and earlier) after a while, IPv6 ND neighbor solicitation messages fail to arrive. I guess it is a bug in multicast filtering. But I have no way to find out.
Result is loss of IPv6 connectivity.
How do you determine they fail to arrive?
Do you have a trace from other hosts? No filters in the network?
In these cases, which NIC/driver are you using? What happens if you turn on PROMISC/MONITOR mode on the interface?
I have a setup with a tp-link running openwrt as router, as small switch behind the wired ethernet ports of the tp-link and then a few freebsd hosts.
One recently built ryzen2 system has an re0 interface. This system works perfectly fine under 11.2 and fails under 12.0 betas and RC1/RC2. With boot environments I can easily switch between the two versions.
When I reported this bug, I just upgraded a Thinkpad x201 (em0 interface) to RC2.
The thinkpad didn't report any neighbor solicitations even with tcpdump. At the same time, on the tplink they were visible in tcpdump.
Also a ping6 from the ryzen2 system (running 11.2) to the thinkpad failed.