Don't exactly know how to explain this one.
Clicking on the Chaos Altar right as I'm about to get hit causes the HP bar above my character to display as if I were at half health. This also makes the damage dealt by the enemy npc not appear.
Also, I've encountered this half health bug thingymajig before while under attack at rock crabs. Don't know how it happened, but I think this hp bug can be reproduced with other npcs/items.