Agonizer sometimes immune to damage?

I am farming Agonizer 9000 with Fl4k using a non GiTM Fade Away build. I shoot him with 3 rockets from my Quickie and like 80% of the times it deals no damage. 2 on 3 shots on average display immune both for the gun damage and the various ASE damages I might be dealing when I detonate the rockets. Other weapons seen to be fine.
Is this specific to this boss? In A LOT of hours I’ve never encountered this behaviour anywhere else…