I've experienced this before. I always just assume that I might have gotten one-shotted. But I mean this is in high GRs where things really can one-shot you. I don't know the level you are fighting Belial at but judging from your gear you have very low Vit and no mitigation, if you are playing T7+ it's very possible you are getting one-shotted. SC is supposed to be squishy (add to that you don't even have the build complete).
Having said that, this "double-one-shot" doesn't happen enough for me to be that concerned.