The item was updated to give mana regen per 100 points of lightning protection instead of armor. However, when you do anything that updates armor values (equip gear, toggle juggernaut stance), it seems the character sheet calculations revert to using mana regen per 100 points of armor. Guessing there’s a separate function that manages updates of effects. Not sure if this is just a visual bug or an actual bug
edit: also taking off urzil’s pride seems to update mana regen to use armor (granting mana regen buff on character sheet without the item on). However this seems like a minor issue since you probably would want to wear a chestpiece and the mana regen is wiped when another chestpiece is equipped