Not sure if this is a bug or just wrong text:
From the game guide
When a character has any % of Damage Taken From Mana Before Health, a portion of the damage will be dealt to their mana instead of health. However, if they are at 0 or negative mana, then all damage is applied to health instead. Damage Taken From Mana Before Health stat does not protect ward, similar to endurance.
Flame Ward : Mental Aegis - Mana now absorbs damage while Flame Ward is active. Each point of mana shields 5 health and ward. Percent Of Damage Absorbed: 15% per point.