It appears that Viscidus gets to perform actions when you shatter him. In our logs I saw him melee, poison shock and poison bolt volley in the brief instance before he turns into blobs. Not all of those every time, but I saw melee + poison shock happen in the same shatter and when he died, he melee’d, poison shocked AND poison bolt volleyed. In other words he gets to do anything he can do during the blip in time. There should be no way he should get any type of activity from when he’s frozen until when he reforms (as long as you shatter him).
In thinking about it some more… this is very similar to other cc type abilities where mobs “save up” attacks and get to unleash them instantaneously when said cc ends.