My guess on this, and a total guess, is that if a council member casts hydra then dies, if you revive that council member then the hydras it spawned are no longer 'seen' by the game and therefore don't delay the next wave until they dissipate. Could be 100% wrong, so don't quote me on this.
Done some more testing yesterday and though it's often hard to tell who casts what, the above seems to be the case.
Ha, quoted you anyway
