Neordla's Talisman giving 0 mana
 

I just got a Nerodla's Talisman, I currently wear a neck that is 5 wis, 50 mana. When I put on the Talisman I should have 20 less mana. But currently i am 40 mana down. Seems the talisman is giving +0 mana instead of giving +20 mana

Resolving as the math doesn't make sense, would want more details on class, active buffs, and other neck item. Repost if you have better documentation.