Nightmare Raids Weapon X. Why am I taking burst damage?

This is Nightmare Raids Weapon X third boss, middle path. This happened 3 times, after the first 2 we recorded the fight. I have no idea why IM would be taking burst damage here. I’ve read the nodes a few times and don’t see a reason why this would happen. I can’t image there’s something in Weapon X’s kit that would do this. Can anyone help explain this? Or is this a bug?
https://youtu.be/ku-SB3an3hk

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