do you have any address pool (not named or delegated) or there’s no addresses at all?
I set up different pools with different names to do the tests, and the behavior is exactly what I said, if I send only the Delegated-IPv6-Prefix-Pool by radius it simply ignores it, however if I send the Stateful-IPv6-Address- Pool and the Delegated-IPv6-Prefix-Pool then it selects the IPs of the informed pools