Nihilus w/ Nihilus lead doesn't regain health after Annihilate

This ability works EXACTLY as it should, in my opinion, in the assault battle-places of power. This assault battle is actually coming up in a couple of days. Try it out. It's incredible....BUT..... I've never understood why annihilate doesnt give that character's max health to Nihilus in other areas of the game such as arena, GW, or raids.
Play this assault battle and use annihilate. Watch the damage you can absorb . Use it in other areas of the game and you will notice that this max health bonus does not happen.