On a run of Heroic Anointed Blades, I chose the Mark of the Viper 3 Treasure for Lady Anacondra. In the following fight, King Mukla and King Krush have the buff applied to them twice while Anacondra has the buff applied once.
Opponents: Furbolg Pathfinder (28), Furbolg Ursa (28), Furbolg Warrior (28)
King Mukla - Level 30 - Banana Frenzy 5 - Dinner Time 3 - Primal Power 2 - Radioactive Bananas 3 equipped - Hunting Party Treasure at +6 attack
King Krush - Level 30 - Apex Predator 5 - Terrify 2 - Devilsaur 3 - Flaming Claws 2 equipped - To the Death! Treasure
Lady Anacondra - Level 30 - Serpent’s Bite 5 - Mend Beast 5 - Nightmare Viper 5 - Toxic Venom 4 equipped - Mark of the Viper 3
I also confirmed their abilities are activating at 4 speed faster for Mukla and Krush but only 2 speed faster for Anacondra
Edit Issue is still active in the next fight as described
Acquiring the Treasure on a separate run causes the same problem
It looks like the Mark of the Viper treasure is applying to beasts even if Lady Anacondra is on the bench but only once. My King Krush has one level of the buff despite Anacondra not being out in this fight.