@mloiterman Make a /128 Virtual IP address on your WAN in on of the /64s you want to route downstream. Make a WAN rule passing ICMP6 to that address. Ping it from the outside. Until that works you're not going to be able to route it downstream.
pfSense is doing what it's supposed to be doing with the /64s on a tracked inside interface. That doesn't mean it's a new delegation. Just that dhcpd is adding that prefix to that interface from the delegation.
Go to System > Advanced, Networking and enable the debug on dhcp6c. Then edit/save WAN. Then go to Status > System Logs, DHCP and filter on Process: dhcp6c. See what is there. That should show you the prefix that was assigned.