I have sent the following to support (Ticket#2016021566000655). Posting here for general awareness.
Summary: The change to the DHCPv6 Client making Rapid Commit the default, in 6.34, has broken "prefix-hint". The hint request never gets sent and the requested prefix size cannot be obtained.
add add-default-route=yes interface=ether1 pool-name=testv6 prefix-hint=::/60 request=prefix
In 6.33, the "hint" is sent in the solicit message (Prefix length field).
In 6.34, the change to Rapid Commit, the hint is never sent.
Is there a way to disable Rapid Commit that I am not seeing???