admin wrote: But if the workaround is assuming an address in spite of what the server is telling you then that's breaking the protocol, no?

Not if you ask before it allocates the /56 prefix. The rule is ask for the last /56 when you send the DHCPv6 solicit and then use whatever is given when you get a delegated prefix. That way you do not break the protocol - rather you are using it as intended.

I've solved (worked around) the problem by moving to IDnet. Now I have a static IPv4 and IPv6 address.