Just killed Zi Thuuli in Dreadspire Keep. This time, he spawned again instantly upon his death. There was no zone shout, and a chest dropped upon the death of both.
The only thing I did differently than I normally do, is I left his first round of adds (obsidian gargoyles) alive all the way through the fight. Didn't feel like killing them, and thought I'd just burn it down. Don't know if that screwed something up, or not, but felt it's worth mentioning.