Pretty sure its not a random effect or bug. It gains revenge abilities after you take out its minions. If you kill them quick enough it won’t be able to use revenge effects on its bigger moves like that nullifying impact that targets everyone when revenge effect is active.
Everytime you kill a minion it gets the revenge effects. so you kill them both 3 times across each round of the wave. So you’ll be getting hit with at least 3 revenge attacks bare minimum. If you don’t kill the minions on the same turn then it gets more revenge effects.
that’s not true taunt has exactly the effect it should have to prevent the attack of a random target … that’s why there are also dinosaurs that are immune to taunt which does not apply to grogobax so this is really a bug … so as there is no one with Ceramagnus because he attacks the taunt ignories