Tested it with my friends. Definitely is not hurting mobs below 50%, and definitely not healing my allies when they are below 50%.
I tested it numerous times on both living and undead mobs in the King Forest and various instances.
I am above 50% during all tests.
Also, the mana it is using is showing up as red numbers making it look like incoming damage.
Am I missing something, or is it bugged?