illumos is not responding to Neighbor Solicitations sent to the solicited-node multicast address (icmpv6)
(I've seen the behavior for both OmniOS and OpenIndiana).
illumos sends DHCPv6 Solicit messages. DHCPv6 server responds first with a few Neighbor Solicitation messages sent to the illumos host's solicited-node multicast address. illumos does not respond, so the Neighbor Discovery is deemed to have failed. This repeats every two minutes ad nauseum.
illumos is also, however, sending intermittent, unsolicited Neighbor Advertisement messages to the link-local multicast every five minutes.
The two intermittent events eventually align so that an unsolicited Neighbor Advertisement happens close enough to the Neighbor Solicitation that the DHCPv6 server accepts the response. Afterward, the DHCPv6 server sends a DHCPv6 Advertisement, and the illumos host finishes the address conversation and assignment.
With seven tested hosts, the time interval to acquire a DHCPv6-provided IPv6 address ranged from 70 minutes to eight hours.