I am very familiar with Finley’s mechanic and I don’t think he’s working correctly against the season 3 enemies. In the video below all 3 enemies have the new “attack stack” buff active. Finley’s special should have consecutively targeted all enemies, but instead he stops after the first attack.
From what I understand, ‘stacks’ are now a new type of status effect. If you look at the cards for some previous heros with dispel everything like Ameonna, it now says: “Remove all status effects and stacks”. So complicated, yes, but stacks are now separate from buffs and impacted separately from buffs. With the case of Mireweave at least, the stack is permanent. Much like the attack boost stack that defenders get in that particular war rule.