Cap IW and Mr. Negative Interaction Question

One of my alliance mates is taking damage while fighting Mr. Negative with Cap IW. He tried it in a quest and practice mode with the same result. Any ideas why he’s taking the damage? I’ve attached a screenshot of the practice fight. 

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Comments
He attemps nullify one every single hit when he got a charge. It doesn't matter that this only nullifies unstoppable buffs.
You'll see the same problem with Thing after he uses a Special Attack - his Stopping Power ability is a Nullify, too.
Or Kamala Khan if she lands a Special with 2+ Furies.