So I suspect the reason for this behavior is that there is no reason why
you'd want to run dhclient in IPv6 mode before you've brought up the
interface and parsed the router advertisement.

That's because unlike IPv4, IPv6 configuration is mostly done through
router advertisements which happen entirely outside of DHCPv6. The
content of that router advertisement then dictates whether DHCPv6 should
be done and if so, in which mode.

So the way things are expected to happen is:
 - Bring up the interface
 - Do router solicitation and get a router advertisement
 - Parse the router advertisement, looking for the other-config (O) and managed 
(M) flag.
 - If the managed bit is set, then trigger dhcpv6 in stateful mode
 - If the managed bit isn't set and other-config bit is set, then trigger 
dhcpv6 in stateless mode
 - If neither are, then use the RDNSS and DNSSL information from the 
advertisement to configure DNS

Triggering dhclient in stateless or even stateful mode before getting a
router advertisement may succeed but may also run into a race condition
as the gateway and prefix length may not have been received by the time
the DHCP dialog is over, leading to invalid configuration.

** Changed in: isc-dhcp (Ubuntu)
       Status: New => Invalid

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  'dhclient -6 -S' does not bring interface up

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