Release 12.1 on amd64 real hardware.
Have both non-vnet and vnet jails using fc00::2 format for local only access between host and jails.
The jls command only shows the first ipv4 address of multiple ipv4 addresses assigned to a jail.
The jls command DOESN'T show any running jail that only has a ipv6 address.
True, the default output of jls isn't very useful in the days of IPv6. But it's a machine-readable format, and won't change.
You can see all the IPv4 and IPv6 addresses with "jls -v". You might also be interested in "jls -s" and "jls name ip4.addr ip6.addr".