pfSense

To summarize:

I have one WAN and one LAN interface. The LAN (igb) interface also has an OPT interface for VLAN 2 named GUESTVLAN (opt1), which is set to track the WAN interface with prefix ID 2. The WAN interface is configured for DHCPv6 and to request a prefix of size /60.

Router advertisements are configured for both LAN subnets. In the LAN subnet, I can see a DNS server being advertised with a prefix ending in 0, and in the GUESTVLAN subnet, I can see a DNS server being advertised with a prefix ending in 2, as expected.

However, the problem is that both subnets are advertising the same address prefix for use via SLAAC. I first confirmed this with Wireshark, and then I loaded /var/etc/radvd.conf and saw that the router advertisement daemon is set to advertise the same prefix on both networks. I'm including a copy of the automatically generated config file here.