I noticed that having Emperor Palpatine lead sith allies recover 20% of their max health when they inflict a debuff on their turn. Since Darth Nihilus inflicts a damage over time at the start of his t...
I think it is because both are happening simultaneously at turn start. Seems simultaneously things cause weird effects. Cracking doom can crit under dn lead Advantage and buff immunity can apply to sith at the same time.