Smash/Discharge/Wither/Flame Sweep/Sweeping Slash > Healing threat
Any of those moves done at the moment the adds appear (i.e. them jetting down from the ship) will register with them and latch them onto the tank. They will only pull off the tank after that if a DPS either 1) hits them with an AoE or 2) targets an add and starts attacking it. Healing will not override the tanks AoE if they continue to use it.
You obviously dont know any good healers