Hydra - unruned, as well as Blazing Hydra and Mammoth Hydra do not trigger the 6piece bonus, and neither do Blizzard - Apocalypse and Wave of Force - Heat Wave.
At least on Heat Wave I think it is a bug, though Blizzard - Apocalypse, unruned Hydra an the initial hit of Blazing Hydra should proc it as well imo.
I can understand why they might not want to have Blazing Hydra and Mammoth Hydra procing the 6piece bonus, because it would probably cause server issues with all the Area Damage procs going of.
Not sure if Blizzard would also cause so much trouble for the servers, but it already is in the Hydra set.