I'll need some time without users on my net to do some proper testing. But I did do a packet capture, and the ICMP6 flow looked normal. When my pfSense transmitted packet from the SLAAC GUA address to the fe80 default gateway, they were simply blackholed.
When pinging link local addresses, you have to specify the interface with the -I option. You could also try capturing the full DHCPv6 sequence and posting the file here.