Could be the ambiguity of the Arcanist role. Who knows...? Maybe the ACN had queued as a SCH but dropped back to ACN for solo-hunting, when the Duty popped?

I did a Halatali run back in October where our CNJ quit after the first pull, and we finished the entire run with just 3 people. Me as GLD; a sync'd BRD, and the Arcanist dual-role as DPS *and* Healing the whole way!