The Vertex Tribal Mask Corruption Modifier Bug

Got a Corrupted Vertex Tribal Mask. The new modifier from corruption " x% Increased Mana Cost Efficiency " it does not reduce the mana cost per spell, it raise the MANA AND LIFE cost per spell.

When use with the Idol of Eeshta with new modifier from the corruption, new modifier over write Idol of Eeshta's 12% Increase Cost Efficiency. Idol of Eeshta's effect doesn't work, it did not reduce mana and life cost per spell. Spell and Life cost per spell, it actually raised per new corruption modifier's at the exact %.

To verify, I switch to a rare helm with Abyss's Desecrated Modifier " x% increased Mana Cost efficiency " it worked as described. It reduce the mana cost per spell, life cost stay the same.


Update:

Purchase a new Corrupted Vertex Tribal Mask with +x Level of All Skills only. No rune in the socket. After Equip with Mask, Mana and Life cost per spell went up 30%.

Slot in the Idol of Eeshta, it did reduce the Mana and life cost per spell, but after the 30% increase.

Looks like it's a Mask Corruption Issue.