Sometimes when killing this boss twice in a row, the 2nd kill won’t yield credit nor provide a choice of blessings. It does still drop loot though.
I belief this happens because the necrotic ring attack around him can persist between levels. It’ll even show if you do regular monolith maps sometimes. Something about that makes you no receive credit for killing it the 2nd time. At least that’s my best guess.
EDIT: the necrotic ring persisting between levels is much more likely to occur if you’ve died to the necrotic blast.
EDIT 2: Also happens when an Umbral Blade is still spinning in the boss room from your 1st visit and re-entering a 2nd time before they expire