pfSense

If you have a downstream interface configured to use a dhcpv6-pd assigned prefix (my isp gives a /56) and you have VIPs configured on the same interface (ie. fd00:dead:beef::1/64 or any prefix for that matter, even if its an actually routable one), IPv6 forwarding stops working completely for that interface.

I have rules in place that leverage alias objects containing both the pd-assigned addresses and the VIP networks since connectivity policy logic doesnt change regardless of the network used for communicating.

Reproductible: always
Workaround: remove VIPs from interface and reboot, assign vips again after reboot.
Notes: I have not tried if forwarding stops working for all interfaces or just the ones that have VIPs assigned to them. This setup is very common for segments that have a public routable prefix but also need ULA addressing for internal connectivity (this kind of setups are described in almost every deployment example published for ipv6 since 1996).

My bug https://redmine.pfsense.org/issues/8276 touches on this same issue and propably needs to be addressed for the fix at the same time since im guessing the root cause originates from the same stem.