"show dhcpv6 server leases" shows outdated (very old) leases from "/opt/vyatta/etc/config/dhcpdv6.leases".
Instead it should use "/config/dhcpd6.leases" which is used by dhcpd.
ps shows dhcpd using /config/dhcpd6.leases:
/usr/sbin/dhcpd -q -6 -lf /config/dhcpd6.leases -cf /etc/dhcp/dhcpd6.conf -pf /var/run/dhcpd6.pid
(edit): fixed typo in filename