Mercenary leap slam damage bug

I use a armour stacker Smoulderstrike with molten strike, vaal molten strike and leap slam. The leap slam is supported by brutality.

Once the mercenary leap slam onto an enemy and perfectly overlap with it, all following attacks from himself Cannot Deal Any Damage! (The hit sound effect still exists). Only when the "kind" enemy move or I forced him to move (long distant teleport or convocation for mercenary), he can deal damage to the enemy again. But if I transported him to a wrong distance, he would just bother me via leaping onto the enemy over and over again.

Not sure whether other mercenaries with leap slam have the same problem, and also not sure whter brutality support lead to this, cause it's so difficult to meet a mercenary with correct skills.