I decided to share my setup for SLAAC on jail vnet.
1. Create bridge and epair interface in /etc/rc.conf
#Configure bridge interface for jails vnet
#epair0 - jail interface
cloned_interfaces="bridge0 epair0" #create bridge and epair
ifconfig_bridge0="ether xx:xx:xx:xx:xx:xx addm re0 SYNCDHCP"...
I am experiencing this odd issue when I restart one of my jails, which operates as the IPv6 router, does not respond to IPv6 packets, even though tcpdump reveals that it receives them. It isn't until it initiates a traffic with other hosts that it starts to respond. IPv4 works just fine.