Lulu Ult is a Heal - So red buff should prevent it
Gaining health is what healing IS. Before her ult, you have less health and after her ult you have more health. That's the definition of a heal.
If her ult merely increased your max health bar, then you wouldn't get any health. The fact that you get more health means its a heal.
Red buff should be able to prevent Lulu's heal. There are plenty of items that completely negate a champion altogether, why should this be an exception?