Darth Nihilus not reviving health from Annihilate

Nihilus doesn't get health steal from Annihilate, he gains Max health. So if your Nihilus has 20k max health, and target had 30k, your Nihilus now has 50k max health. You may not notice the bar change because there are so many tics. So the percentage of health stays the same, but he is definitely gaining max health.

The reason he recovers health from raid bosses is because instead of defeating bosses, he deals 99,999 damage. So due to health steal he will fill his health back up.